Link to the PaymentRequest protocol buffer

Since developers need this resource, I thought it would be better to add a direct link to the relevant file in Bitcoin Core source code so they can download the file without searching.

I had to use \<!--noref--\> to prevent autocrossref.rb from corrupting the link. In case someone here is a regex master, it would be nice to find a way to prevent the plugin from touching urls. I gave it a try, but ruby and most regex don't support wildcards in negative look-behind.