Add 'Documentation' section to 'Contributing' docs

There have been a bunch of patches proposed lately to generated
config files.  This patch adds a section to the Contribution
guidelines explaining where to make changes for various types of
docs.

Change-Id: Iad971e89f9f1a9d534f6040d9516c5be3efd2926

doc/source/contributing/documentation.rst

@@ -0,0 +1,40 @@
+Documentation
+=============
+
+There are a few different kinds of documentation associated with Glance to
+which you may want to contribute:
+
+* Configuration
+
+  As you read through the sample configuration files in the ``etc`` directory
+  in the source tree, you may find typographical errors, or grammatical
+  problems, or text that could use clarification.  The Glance team welcomes
+  your contributions, but please note that the sample configuration files are
+  generated, not static text.  Thus you must modify the source code where the
+  particular option you're correcting is defined and then re-generate the conf
+  file using ``tox -e genconfig``.
+
+* Glance's Documentation
+
+  The Glance Documentation (what you're reading right now) lives in the source
+  code tree under ``doc/source``.  It consists of information for developers
+  working on Glance, information for consumers of the OpenStack Images APIs
+  implemented by Glance, and information for operators deploying Glance.  Thus
+  there's a wide range of documents to which you could contribute.
+
+  Small improvements can simply be addressed by a patch, but it's probably a
+  good idea to first file a bug for larger changes so they can be tracked more
+  easily (especially if you plan to submit several different patches to address
+  the shortcoming).
+
+* User Guides
+
+  There are several user guides published by the OpenStack Documentation Team.
+  Please see the README in their code repository for more information:
+  https://github.com/openstack/openstack-manuals
+
+* OpenStack API Reference
+
+  There are some Glance-related API quick-reference guides published by the
+  OpenStack Documentation Team. Please see the README in their code repository
+  for more information: https://github.com/openstack/api-site

doc/source/contributing/index.rst

@@ -12,6 +12,7 @@ Policies
    :maxdepth: 3
 
    blueprints
+   documentation
 .. bugs
    contributor-onboarding
    core-reviewers