Allow personal overrides to psqlrc

I wanted to add my own customizations after reading @mike-burns' post on [`psqlrc`](http://robots.thoughtbot.com/an-explained-psqlrc), but my brief research indicates that there's not a clean way to sourcea file only after checking to see if the file exists from psql. I think the best solution is to include a blank `psqlrc.local` file so that it can always be sourced, but for users with no overrides it supplies no settings.

Adding a blank `psqlrc.local` file that users can override with their own custom
dotfiles, which will allow users to override the thoughtbot-provided defaults
(while continuing to receive upstream changes) and make their own
personalizations.
@gylaz, you're right, they are not needed. While changing the docs for the initial install I didn't want to remove this info, but if there's consensus around deleting it, happy to do so.