Clarified how to create a good pull request.

Updated CONTRIBUTING.md with some tips.

Change-Id: Id377621a98804293837fda25ef0758cf988eea37

CONTRIBUTING.md

@@ -23,6 +23,20 @@ frustration later on.
 All submissions, including submissions by project members, require review. We
 use Github pull requests for this purpose.
 
+Some tips for good pull requests:
+* Use our code
+  [style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html).
+  When in doubt, try to stay true to the existing code of the project.
+* Write a descriptive commit message. What problem are you solving and what
+  are the consequences? Where and what did you test? Some good tips:
+  [here](http://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message)
+  and [here](https://www.kernel.org/doc/Documentation/SubmittingPatches).
+* If your PR consists of multiple commits which are successive improvements /
+  fixes to your first commit, consider squashing them into a single commit
+  (`git rebase -i`) such that your PR is a single commit on top of the current
+  HEAD. This make reviewing the code so much easier, and our history more
+  readable.
+
 # The small print
 Contributions made by corporations are covered by a different agreement than
 the one above, the Software Grant and Corporate Contributor License Agreement.