Reduce chance of developer not using thin
Reduce chance of developer not using thin

The context for this README change is that I spent at least 30-60 minutes diagnosing the following:

1. zuul on my local machine was failing under moderate load, because
2. rackup was using WEBrick rather than thin, because
3. bundle wasn't pulling in `development` dependencies, because
4. `bundle install` was being treated as `bundle install --without development`, because
5. of a bug bundle explained in https://github.com/bundler/bundler/issues/2862

The fix is to either:
* use the workaround of the bug above: `bundle install --without nothing`, or
* explicitly start `thin`

This commit's README change suggests doing both of those.

blinkboxbooks/auth-server