Readme fixes: correcting errors

Correcting instructions for installing Python versions (removing "download and unpack the source").

Fixing links to #pyenv-shell, #pyenv-local, and #pyenv-global - linking them to the appropriate sections of the COMMANDS.md page.

README.md

@@ -111,19 +111,19 @@ When you execute a shim, pyenv determines which Python version to use by
 reading it from the following sources, in this order:
 
 1. The `PYENV_VERSION` environment variable (if specified). You can use
-   the [`pyenv shell`](#pyenv-shell) command to set this environment
+   the [`pyenv shell`](https://github.com/yyuu/pyenv/blob/master/COMMANDS.md#pyenv-shell) command to set this environment
    variable in your current shell session.
 
 2. The application-specific `.python-version` file in the current
    directory (if present). You can modify the current directory's
-   `.python-version` file with the [`pyenv local`](#pyenv-local)
+   `.python-version` file with the [`pyenv local`](https://github.com/yyuu/pyenv/blob/master/COMMANDS.md#pyenv-local)
    command.
 
 3. The first `.python-version` file found (if any) by searching each parent
    directory, until reaching the root of your filesystem.
 
 4. The global `~/.pyenv/version` file. You can modify this file using
-   the [`pyenv global`](#pyenv-global) command. If the global version
+   the [`pyenv global`](https://github.com/yyuu/pyenv/blob/master/COMMANDS.md#pyenv-global) command. If the global version
    file is not present, pyenv assumes you want to use the "system"
    Python. (In other words, whatever version would run if pyenv weren't in your
    `PATH`.)
@@ -214,7 +214,7 @@ easy to fork and contribute any changes back upstream.
         $ exec $SHELL
 
 5. **Install Python versions into `$PYENV_ROOT/versions`.**
-   For example, to install Python 2.7.8, download and unpack the source, then run:
+   For example, to download and install Python 2.7.8, run:
 
         $ pyenv install 2.7.8