Improve the README
Improve the README

The README needed a little love. Primarily, I wanted to avoid
recommending `sudo gem install` as that's widely considered not to be a
best practice. Using a version of Ruby installed with rbenv is a better
way to avoid permission errors.

 * Recommend using rbenv to install a modern version of Ruby.
 * Avoid recommending `sudo` to install gems.
 * `bundle install --path ...` is deprecated, and isn't really necessary
   anyway. A simple `bundle install` is fine.
 * Clarify update instructions.

opensourcecatholic/opensourcecatholic.github.io,opensourcecatholic/opensourcecatholic.github.io