CONTRIBUTING: Add a section on coding style

Document that we now define our coding style with `clang-format`.

Change-Id: I6dea8c3816f277b0e56e074ccd6c64a2cfeb094c

CONTRIBUTING.rst

@@ -24,6 +24,19 @@ KWSys has no independent issue tracker.  After encountering an issue
 Otherwise please report the issue to the tracker for the project that
 hosts the copy of KWSys in which the problem was found.
 
+Code Style
+==========
+
+We use `clang-format`_ to define our style for C++ code in the KWSys source
+tree.  See the `.clang-format`_ configuration file for our style settings.
+Use ``clang-format`` version 3.8 or higher to format source files.
+See also the `clang-format.bash`_ script.
+
+.. _`clang-format`: http://clang.llvm.org/docs/ClangFormat.html
+.. _`.clang-format`: .clang-format
+.. _`clang-format.bash`: clang-format.bash
+
+
 License
 =======