docs/contributing: make contribution information more accessible

Try to make the contribution information more accessible to new users, by
giving a bit more detail about the different steps in making contributions.

Some rework of the original patch done by Thomas De Schampheleire:
- don't give the impression that Bitbucket is mandatory
- don't recommend creating feature branches for regular development
- reword/cross-reference section on contribution flow

docs/contributing.rst

@@ -28,7 +28,7 b' for more details.'
 Getting started
 ---------------
 
-To get started with development::
+To get started with Kallithea development::
 
         hg clone https://kallithea-scm.org/repos/kallithea
         cd kallithea
@@ -41,8 +41,41 b' To get started with development::'
         gearbox serve -c my.ini --reload &
         firefox http://127.0.0.1:5000/
 
-You can also start out by forking https://bitbucket.org/conservancy/kallithea
-on Bitbucket_ and create a local clone of your own fork.
+If you plan to use Bitbucket_ for sending contributions, you can also fork
+Kallithea on Bitbucket_ first (https://bitbucket.org/conservancy/kallithea) and
+then replace the clone step above by a clone of your fork. In this case, please
+see :ref:`contributing-guidelies` below for configuring your fork correctly.
+
+
+Contribution flow
+-----------------
+
+Starting from an existing Kallithea clone, make sure it is up to date with the
+latest upstream changes::
+
+        hg pull
+        hg update
+
+Review the :ref:`contributing-guidelines` and :ref:`coding-guidelines`.
+
+If you are new to Mercurial, refer to Mercurial `Quick Start`_ and `Beginners
+Guide`_ on the Mercurial wiki.
+
+Now, make some changes and test them (see :ref:`contributing-tests`). Don't
+forget to add new tests to cover new functionality or bug fixes.
+
+For documentation changes, run ``make html`` from the ``docs`` directory to
+generate the HTML result, then review them in your browser.
+
+Before submitting any changes, run the cleanup script::
+
+        ./scripts/run-all-cleanup
+
+When you are completely ready, you can send your changes to the community for
+review and inclusion. Most commonly used methods are sending patches to the
+mailing list (via ``hg email``) or by creating a pull request on Bitbucket_.
+
+.. _contributing-tests:
 
 
 Running tests
@@ -114,6 +147,8 b' To analyze performance, you could instal'
 
 .. _pytest-profiling: https://github.com/manahl/pytest-plugins/tree/master/pytest-profiling
 
+.. _contributing-guidelines:
+
 
 Contribution guidelines
 -----------------------
@@ -153,6 +188,8 b' For now we just have one official branch'
 that it can be (and is) used in production. Experimental changes should live
 elsewhere (for example in a pull request) until they are ready.
 
+.. _coding-guidelines:
+
 
 Coding guidelines
 -----------------
@@ -272,3 +309,5 b' Thank you for your contribution!'
 .. _Hosted Weblate: https://hosted.weblate.org/projects/kallithea/kallithea/
 .. _wiki: https://bitbucket.org/conservancy/kallithea/wiki/Home
 .. _DebugBar: https://github.com/TurboGears/tgext.debugbar
+.. _Quick Start: https://www.mercurial-scm.org/wiki/QuickStart
+.. _Beginners Guide: https://www.mercurial-scm.org/wiki/BeginnersGuides