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.
Re-wording this would be an improvement. How about something like:
```
* Exclude the `README.md`, `LICENSE`, and `Brewfile` files, which are part of the `dotfiles` repository but do not need to be symlinked in.
```