Update README to include reminder about config.consider_all_requests_local

**Problem:**

  * I took forever to upgrade `better_errors` because the last time I tried it broke, I freaked out, and didn't dig into why. Ashamed that I was using such an old version, I dug into where things stopped working for me and realized [this commit](https://github.com/charliesome/better_errors/commit/7d978a220af774fb4967bc5c13d1998987188525) introduced a check to verify that `config.consider_all_requests_local = true` in `development.rb`. So I scampered into that file, made the change, and verified the fix. So I wanted to include a note in the README about it for others.