Fixed #17933 -- Added intro note to writing-documentation.txt

This section explains how writers can craft their documentation changes

in the most useful and least error-prone ways.

Getting the raw documentation

-----------------------------

Though Django's documentation is intended to be read as HTML at

http://docs.djangoproject.com/, we edit it as a collection of text files for

maximum flexibility. These files live in the top-level ``docs/`` directory of a

Django release.

If you'd like to start contributing to our docs, get the development version of

Django from the source code repository

(see :ref:`installing-development-version`). The development version has the

latest-and-greatest documentation, just as it has latest-and-greatest code.

Generally, we only revise documentation in the development version, as our

policy is to freeze documentation for existing releases (see

:ref:`differences-between-doc-versions`).

Getting started with Sphinx

---------------------------