Updated instructions for OS X .app editors

I've been using It's All Text for years on Linux and OS X, even Windows when former employers insisted.  I actually solved the OS X thing a while back, I just didn't realise it was such a big deal.  The instructions I've added to the readme, with an example taken from my own use covers exactly what needs to be done (.app files aren't files, they're directories and the binary people need to call are a couple of levels deeper in).

BTW, I'm writing this pull request using Emacs and It's All Text loaded just the way I've described.  ;)