Require vc-git, not vc
Don't autoload find-file-hook addition, update docs

It's considered bad practice for a library to unilaterally modify hooks:
if this library were bundled with Emacs, it would not be expected to
auto-enable itself.

This commit makes users enable bug-reference-github explicitly, and
outlines a couple of alternative methods which might suit them.
Fixed MELPA url.