Ruby

Fleshed out Ruby conventions further, with sections on Rails and Bundler. [fixes #2]
FWIW, my approach in [Checkout](https://github.com/MonkDev/mchk/blob/dev/Gemfile)/[Donate](https://github.com/MonkDev/mdnt/blob/dev/Gemfile) (inspired by that article a while back) has been:

1. Always lock Rails.
2. Don't lock anything else unless I need to (and then, as pessimistically as I can get away with).
3. Document why something is locked.