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).

As a result of this change, `.psqlrc` requires that a `.psqlrc.local` exist in
the home folder because `psql` cannot do conditional sourcing of files in the
style that we use in other foundation dotfiles. Touching the file to ensure it
exists (even if blank) prevents `psql` from exiting with an error if the user
does not provide their own custom configuration.

Additional changes:
* Updating the documentation with regard to which files get excluded.