README: Add guidelines on how to contribute patches

Ben Avison pointed out here:

   http://lists.freedesktop.org/archives/pixman/2013-January/002485.html

that there isn't really any documentation about how to submit patches
to pixman. This patch adds some information to the README file.

v2: Incorporate some comments from Ben Avison
v3: Change gitweb URL to cgit

README

@@ -1,22 +1,116 @@
-pixman is a library that provides low-level pixel manipulation
+Pixman is a library that provides low-level pixel manipulation
 features such as image compositing and trapezoid rasterization.
 
-All questions regarding this software should be directed to the pixman
+Questions, bug reports and patches should be directed to the pixman
 mailing list:
 
         http://lists.freedesktop.org/mailman/listinfo/pixman
 
-Please send patches and bug reports either to the mailing list above,
-or file them at the freedesktop bug tracker:
+You can also file bugs at
 
         https://bugs.freedesktop.org/enter_bug.cgi?product=pixman
 
-The master development code repository can be found at:
+For real time discussions about pixman, feel free to join the IRC
+channels #cairo and #xorg-devel on the FreeNode IRC network.
+
+
+Contributing
+------------
+
+In order to contribute to pixman, you will need a working knowledge of
+the git version control system. For a quick getting started guide,
+there is the "Everyday Git With 20 Commands Or So guide"
+
+        http://www.kernel.org/pub/software/scm/git/docs/everyday.html
+
+from the Git homepage. For more in depth git documentation, see the
+resources on the Git community documentation page:
+
+        http://git-scm.com/documentation
+
+Pixman uses the infrastructure from the freedesktop.org umbrella
+project. For instructions about how to use the git service on
+freedesktop.org, see:
+
+        http://www.freedesktop.org/wiki/Infrastructure/git/Developers
+
+The Pixman master repository can be found at:
 
 	git://anongit.freedesktop.org/git/pixman
 
-	http://gitweb.freedesktop.org/?p=pixman;a=summary
+and browsed on the web here:
+
+	http://cgit.freedesktop.org/pixman/
+
+
+Sending patches
+---------------
+
+The general workflow for sending patches is to first make sure that
+git can send mail on your system. Then, 
+
+ - create a branch off of master in your local git repository
+
+ - make your changes as one or more commits
+
+ - use the 
+
+        git send-email
+
+   command to send the patch series to pixman@lists.freedesktop.org.
+
+In order for your patches to be accepted, please consider the
+following guidelines:
+
+ - This link:
+
+        http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#patch-series
+
+   describes how what a good patch series is, and to create one with
+   git.
+
+ - At each point in the series, pixman should compile and the test
+   suite should pass.
+
+   The exception here is if you are changing the test suite to
+   demonstrate a bug. In this case, make one commit that makes the
+   test suite fail due to the bug, and then another commit that fixes
+   the bug.
+
+   You can run the test suite with 
+
+        make check
+
+   It will take around two minutes to run on a modern PC.
+
+ - Follow the coding style described in the CODING_STYLE file
+
+ - For bug fixes, include an update to the test suite to make sure
+   the bug doesn't reappear.
+
+ - For new features, add tests of the feature to the test
+   suite. Also, add a program demonstrating the new feature to the
+   demos/ directory.
+
+ - Write descriptive commit messages. Useful information to include:
+        - Benchmark results, before and after
+	- Description of the bug that was fixed
+	- Detailed rationale for any new API
+	- Alternative approaches that were rejected (and why they
+          don't work)
+	- If review comments were incorporated, a brief version
+          history describing what those changes were.
+
+ - For big patch series, send an introductory email with an overall
+   description of the patch series, including benchmarks and
+   motivation. Each commit message should still be descriptive and
+   include enough information to understand why this particular commit
+   was necessary.
 
-For more information on the git code manager, see:
+Pixman has high standards for code quality and so almost everybody
+should expect to have the first versions of their patches rejected.
 
-	http://wiki.x.org/wiki/GitPage
+If you think that the reviewers are wrong about something, or that the
+guidelines above are wrong, feel free to discuss the issue on the
+list. The purpose of the guidelines and code review is to ensure high
+code quality; it is not an exercise in compliance.