Notes on choosing/optimising JRuby

Opening this to start the discussion after another request for ideas on `JRUBY_OPTS` from @ggayan.

I'd like to expand this with:

* [ ] Specific explanation of why we recommend JRuby
* [ ] Explanation of each of the JRuby/JVM options

Several people have tackled this in the past, including @bklang, @JustinAiken, @polysics, @sfgeorge, @Jared-Prime and probably others. I'd appreciate input from any and all of you :)