"Writing documentation" needs link to check out documentation

[rmattb]

"Contributing to Django" ​https://docs.djangoproject.com/en/dev/internals/contributing/
links to "Writing documentation" ​https://docs.djangoproject.com/en/dev/internals/contributing/writing-documentation/
but there's no mention from there of where to get the code to edit,

I've added a section explaining where to get the code (check out trunk), and why (existing release docs are frozen).
Patch attached. :)

[Adrien Lemaire]

Working with sphinx implies to get the code and look at the docs directory.

But the patch LGTM.

[Thomas Westfeld]

Added new pull request in github

​https://github.com/django/django/pull/116

[Adrian Holovaty <adrian@…>]

In [87ff89d12d109066ff355ed267787d32a6b5aeb7]:

Fixed #17933 -- Added intro note to writing-documentation.txt
Based on initial patch by Thomas Westfeld

[Aymeric Augustin]

This commit contains a reference to a "differences-between-doc-versions" label, but no such label exists in the the docs.

I thought that I had inadvertently removed this label during my extensive changes to the "contributing" section of the docs (#18436), but that doesn't appear to be true. Actually I couldn't find any such label in Django's history:

    % git log -Sdifferences-between-doc-versions
    commit 87ff89d12d109066ff355ed267787d32a6b5aeb7
    Author: Adrian Holovaty <adrian@holovaty.com>
    Date:   Thu Jun 7 11:32:28 2012 -0500
 
        Fixed #17933 -- Added intro note to writing-documentation.txt
        Based on initial patch by Thomas Westfeld

I suppose that some of the intended changes are missing in the final commit. I'm going to add the label at the most plausible location. You may want to check that there are no other missing pieces.

[Aymeric Augustin <aymeric.augustin@…>]

In [6a66fc08b3a1e3ec56e05cd48647060ac3a4da94]:

Fixed #17933 -- Added a missing label to the docs.