(doc) Add documentation commit example to CONTRIBUTING

Without this path applied there is no example for contributing a
documentation only commit to the Puppet repository. This is a
problem because the contributor is left to assume how a commit of
this nature may appear.

The first line is a real life imperative statement with '(doc)' in
place of what would have been the ticket number in a non
documentation related commit. The body describes the nature of
the new documentation or comments added.