Add docs about troubleshooting migrations

Signed-off-by: David Mehren <git@herrmehren.de>

docs/content/guides/migration-troubleshooting.md

@@ -0,0 +1,25 @@
+# Troubleshooting Migrations
+
+In some cases, HedgeDoc might apply migrations without correctly saving the progress.
+In these cases, HedgeDoc will refuse to start with “already exists”-errors like
+`ERROR: type "enum_Notes_permission" already exists`.
+
+To fix these issues, manual intervention in the database is required:
+
+1. Make sure you have a way to edit the database directly. For SQLite, PostgreSQL and MariaDB/MySQL, you can use the
+    respective command-line tools `sqlite3`, `psql` and `mysql`.
+2. Get the name of the failing migration and append `.js` to it.
+    For example, if you encounter this error
+    ```
+    == 20180306150303-fix-enum: migrating =======
+
+    ERROR: type "enum_Notes_permission" already exists
+    ```
+    the name of the failed migration would be `20180306150303-fix-enum.js`.
+3. Make sure HedgeDoc does not run and insert the name into the `SequelizeMeta` table.  
+   Ensure your database shell is connected to the HedgeDoc database. The SQL-statement may look like this: 
+   ```sql
+   INSERT INTO "SequelizeMeta" (name) VALUES ('20180306150303-fix-enum.js');
+   ```
+4. Start HedgeDoc again and observe if it starts correctly. It may be necessary to repeat this process 
+    and insert multiple migrations into the `SequelizeMeta` table.

docs/mkdocs.yml

@@ -37,6 +37,7 @@ nav:
           - S3: guides/s3-image-upload.md
       - Migrate from Etherpad: guides/migrate-etherpad.md
       - Breaking Changes: guides/migrations-and-breaking-changes.md
+      - Migration Troubleshooting: guides/migration-troubleshooting.md
       - Terms of Use Setup: guides/providing-terms.md
   - Configuration: configuration.md
   - Developer: