docs: Adds 'Using _index.md' page

There has been confusion regarding the use of _index.md pages following
the shift to 'Everything is a Page' in Hugo v0.18:

https://discuss.gohugo.io/t/hugo-0-18-ignores-index-md-in-the-section/4896

This has lead to attempts at clarification:

https://discuss.gohugo.io/t/-index-md-pages-in-section-taxonomies-etc-are-not-rendered-using-single-html-template/5466

There is also an open issue regarding improving the docs on this topic:

https://github.com/spf13/hugo/issues/2827

This commit adds a dedicated page explaining how _index.md files fit
into Hugo's template hierarchy, what they can contain and how to render
content from them onto the respective list/terms/home page.

What this commit doesn't do is address confusion over former behaviour
of the 'index.md' files (note the lack of _). Instead it focuses on
clarifying the new behaviour in the hope of rendering the old moot.

Fixes #2827

docs/content/content/example.md

@@ -8,7 +8,7 @@ menu:
   main:
     parent: content
 prev: /content/multilingual
-next: /themes/overview
+next: /content/using-index-md
 notoc: true
 title: Example Content File
 weight: 70

docs/content/content/using-index-md.md

@@ -0,0 +1,118 @@
+---
+aliases:
+- /doc/using-index-md/
+lastmod: 2017-02-22
+date: 2017-02-22
+linktitle: Using _index.md
+menu:
+  main:
+    parent: content
+prev: /content/example
+next: /themes/overview
+notoc: true
+title: Using _index.md
+weight: 70
+---
+# \_index.md and 'Everything is a Page'
+
+As of version v0.18 Hugo now treats '[everything as a page](http://bepsays.com/en/2016/12/19/hugo-018/)'. This allows you to add content and frontmatter to any page - including List pages like [Sections](/content/sections/), [Taxonomies](/taxonomies/overview/), [Taxonomy Terms pages](/templates/terms/) and even to potential 'special case' pages like the [Home page](/templates/homepage/).
+
+In order to take advantage of this behaviour you need to do a few things. 
+
+1. Create an \_index.md file that contains the frontmatter and content you would like to apply.
+
+2. Place the \_index.md file in the correct place in the directory structure. 
+
+3. Ensure that the respective template is configured to display `{{ .Content }}` if you wish for the content of the \_index.md file to be rendered on the respective page. 
+
+## How \_index.md pages work
+
+Before continuing it's important to know that this page must reference certain templates to describe how the \_index.md page will be rendered. Hugo has a multitude of possible templates that can be used and placed in various places (think theme templates for instance). For simplicity/brevity the default/top level template location will be used to refer to the entire range of places the template can be placed. 
+
+If this is confusing or you are unfamiliar with Hugo's template hierarchy, visit the various template pages listed below. You may need to find the 'active' template responsible for any particular page on your own site by going through the template hierarchy and matching it to your particular setup/theme you are using. 
+
+- [Home page template](/templates/homepage/)
+- [Content List templates](/templates/list/)
+- [Single Content templates](/templates/content/)
+- [Taxonomy Terms templates](/templates/terms/)
+
+Now that you've got a handle on templates lets recap some Hugo basics to understand how to use an \_index.md file with a List page.
+
+1. Sections and Taxonomies are 'List' pages, NOT single pages.
+2. List pages are rendered using the template heirarchy found in the [Content - List Template](http://localhost:1313/templates/list/) docs.
+3. The Home page, though technically a List page, can have [it's own template](/templates/homepage/) at layouts/index.html rather than \_default/list.html. Many themes exploit this behaviour so you are likely to encounter this specific use case. 
+4. Taxonomy terms pages are 'lists of metadata' not lists of content, so [have their own templates](/templates/terms/). 
+
+Let's put all this information together:
+
+> **\_index.md files used in List pages, Terms pages or the Home page are NOT rendered as single pages or with Single Content templates.**
+
+> **All pages, including List pages, can have frontmatter and frontmatter can have markdown content - meaning \_index.md files are the way to _provide_ frontmatter and content to the respective List/Terms/Home page.**
+
+Here are a couple of examples to make it clearer...
+
+| \_index.md location                 | Page affected             | Rendered by                   |
+| -------------------                 | ------------              | -----------                   |
+| /content/post/\_index.md            | site.com/post/            | /layouts/section/post.html    |
+| /content/categories/hugo/\_index.md | site.com/categories/hugo/ | /layouts/taxonomy/hugo.html   |
+
+## Why \_index.md files are used
+
+With a Single page such as a post it's possible to add the frontmatter and content directly into the .md page itself. With List/Terms/Home pages this is not possible so \_index.md files can be used to provide that frontmatter/content to them. 
+
+## How to display content from \_index.md files
+
+From the information above it should follow that content within an \_index.md file won't be rendered in its own Single Page, instead it'll be made available to the respective List/Terms/Home page. 
+
+To **_actually render that content_** you need to ensure that the relevant template responsible for rendering the List/Terms/Home page contains (at least) `{{ .Content }}`. 
+
+This is the way to actually display the content within the \_index.md file on the List/Terms/Home page. 
+
+A very simple/naive example of this would be:
+
+```html
+{{ partial "header.html" . }}
+	<main>
+        {{ .Content }}
+        {{ range .Paginator.Pages }}
+			{{ partial "summary.html" . }}
+		{{ end }}
+		{{ partial "pagination.html" . }}
+	</main>
+{{ partial "sidebar.html" . }}
+{{ partial "footer.html" . }}
+```
+
+You can see `{{ .Content }}` just after the `<main>` element. For this particular example, the content of the \_index.md file will show before the main list of summaries.
+
+## Where to organise an \_index.md file
+
+To add content and frontmatter to the home page, a section, a taxonomy or a taxonomy terms listing, add a markdown file with the base name \_index on the relevant place on the file system.
+
+```bash
+└── content
+    ├── _index.md
+    ├── categories
+    │   ├── _index.md
+    │   └── photo
+    │       └── _index.md
+    ├── post
+    │   ├── _index.md
+    │   └── firstpost.md
+    └── tags
+        ├── _index.md
+        └── hugo
+            └── _index.md
+```
+
+In the above example \_index.md pages have been added to each section/taxonomy. 
+
+An \_index.md file has also been added in the top level 'content' directory. 
+
+### Where to place \_index.md for the Home page
+
+Hugo themes are designed to use the 'content' directory as the root of the website, so adding an \_index.md file here (like has been done in the example above) is how you would add frontmatter/content to the home page. 
+
+
+
+