Add CONTRIBUTING file

Author: vitaut

CONTRIBUTING.rst

@@ -0,0 +1,11 @@
+Contributing to fmt
+===================
+
+All C++ code must adhere to `Google C++ Style Guide
+<https://google.github.io/styleguide/cppguide.html>`_ with the following
+exceptions:
+
+* Exceptions are permitted
+* snake_case should be used instead of UpperCamelCase for function names
+
+Thanks for contributing!