Added secion about project-specific settings to Readme.
Adds examples to `.dir-locals.el` README section.

Provies examples for customizing project compilation command string and
project test command string.

Fixes existing example code.

Adds documentation of using `.dir-locals.el` along with defcustom settings.
Merge pull request #395 from joelmccracken/master

Adds examples to .dir-locals.el section in README