Add instructions for one-off wrapper

[milesfrain]

I find a convenience wrapper to opt-in to difft behavior works best for me, so sharing instructions.

The reason for not wanting to fully replace default git behavior is that I sometimes use my .gitconfig on machines without difftastic installed. I also sometimes want to opt-out of difft behavior in order to see whitespace changes, but I couldn't find an easy way to do that once overriding default diff.

I use difft behavior more often than not, so actually use a shorter gt alias instead of gitt, but figured the instructions would be clearer with the longer version.