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

Based on initial patch by Thomas Westfeld

docs/internals/contributing/writing-documentation.txt

@@ -18,6 +18,22 @@ Documentation changes generally come in two forms:
 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
 ---------------------------