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. This time I dug into it and realized [this commit](https://github.com/charliesome/better_errors/commit/7d978a220af774fb4967bc5c13d1998987188525) introduced a check to verify `config.consider_all_requests_local` was set to true. I scampered into my `development.rb` file, changed that and verified that fixed my issue. So I wanted to include a note in the README about it for others.