Add setup issues to documentation

[jameshochadel]

I had two issues during dev env setup that I believe are not documented. Dumping my notes here:

Issue 1

Error on trying bundle install: An error occurred while installing pg (0.19.0), and Bundler cannot continue.

Solution: Install Postgres on the machine (using brew on macOS). Upgrade if installed.

Ubuntu: Also follow so article
http://stackoverflow.com/questions/19262312/installing-pg-gem-on-os-x-failure-to-build-native-extension

Issue 2

Need to create PROJECT_ROOT/tmp/pid directory in order to start the server for the first time. I don't remember what the exact error was, but one of the servers tries to write its PID to this directory and fails if it doesn't exist.