(PDOC-210) add Puppet Strings documentation

Part of the Puppet Strings effort is having an example module out before
version 2.0.0 is released. This adds 'correct' Strings documentation to
this entire module and moves the Reference section of the README to
REFERENCE.md. Also adds '/docs/' to .gitignore, which will need to
happen in the PDK template.

Author: eputnam

.gitignore

@@ -8,6 +8,7 @@
 /.vagrant/
 /coverage/
 /bin/
+/docs/
 /doc/
 /Gemfile.local
 /Gemfile.lock