Add documentation on Java prerequisites

The JAVA_HOME/having the bin folder bit in your path is true everywhere I think, yes (although obviously the example paths here are for Windows). Registry keys are Windows only, but I think this makes that clear.

https://github.com/calabash/calabash-android/blob/4f85a1f8d65dc814656cff649496c5c430ae2c36/ruby-gem/lib/calabash-android/env.rb#L27 is where ths all happens, if that's useful for context.