Add Notes for installation with Homebrew

[simono]

[lelutin]

hey there. I just sent a notice to bup's mailing list about your pull request. this is great, I didn't know it was accessible via Homebrew :)

here's the message I sent, please continue discussion there:

https://groups.google.com/d/msg/bup-list/fd4ly8K_3c8/9yuqXqKhA7wJ

p.s.: bup's usual patch submission is done via the mailing list -- [email protected] -- (open for the public, so no need to be registered) so that people can review changes. but since your change is so simple, I just notified the list about this and we can probably apply your patch without too much fuss ;)