Add custom.el to .gitignore file.

The .dir-locals.el trick changes the custom.el every time it sets the
"safe list of variables" (which doesn't do what it's supposed to because
we're not actually *setting* a safe list of varialbes, we're running
some code and setting up the environment....

Ignoring it keeps the peace.
Add an example .dir-locals.el file.

Add an example .dir-locals.el file that checks for the existence of a
perl local-lib and if it finds one then sets PATH/PERL5LIB
appropriately.