Add Ruby-RNP documentation and link

[ronaldtse]

Ruby-RNP: https://github.com/riboseinc/ruby-rnp

Ruby-RNP documentation: https://www.rubydoc.info/github/riboseinc/ruby-rnp/master/toplevel

Mentioned here: rnpgp/ruby-rnp#6 (comment)

[strogonoff]

The Ruby-RNP docs is just an API reference. As theme docs point out (and I believe) it’s best to instead link to RubyDoc API reference.

However, what we would want to do additionally (if the goal is to promote the project in the industry) is to provide docs that Ruby-RNP is currently much lacking, namely an overview, a getting started guide and a couple of typical usage tutorials.

In this ticket I’m adding the link to Ruby-RNP and creating the foundation for further docs, for now based on README, later adding sample I’m putting together for the post in #10. These new docs would be placed into ruby-rnp repo’s docs/ directory, and subsequently get picked up during rnpgp.com build.