Add PATH caveats to MacTeX Cask

Warns user that they may need to add the mactex `/usr/texbin` directory to their
PATH manually, as the package does not seem to do this or symlink automatically.

Fixes #2277.

See also [this comment](https://github.com/phinze/homebrew-cask/pull/2284#commitcomment-4974770)
on #2284 for more information.
Merge pull request #2284 from colindean/patch-1

Add PATH caveats to MacTeX Cask