(maint) Add COMMITTERS.md and CONTRIBUTING.md

In order to make contributions more accessible to the community,
add COMMITTERS.md and CONTRIBUTING.md so that there are instructions
and resources for those who would like to contribute to the strings
module.
(maint) Remove references to stable in COMMITTERS

Since puppet strings is a very small project, at the moment we
don't need to maintain a master and a stable branch. As a result,
the COMMITTERS.md guide should not contain references to the stable
branch or include information around dealing with both the master
and stable branch. Remove these references so that COMMITTERS is
consistent with our approach to the strings repository.
(maint) Remove reference to other platforms

The "other platforms" is language from puppet where we don't always have
access to various OS and version combinations. Strings is much simpler
and so it isn't really relevant.
Merge branch 'hkenney-maint/master/add_committers_and_contributing_md'

* hkenney-maint/master/add_committers_and_contributing_md:
  (maint) Remove reference to other platforms
  (maint) Remove references to stable in COMMITTERS
  (maint) Add COMMITTERS.md and CONTRIBUTING.md

Closes GH-11