Offer The BFG as an alternative to git-filter-branch

For the time being, I've left the git-filter-branch instructions as they
are, and just offered The BFG as alternative.

I think the Remove-Sensitive-Data article might benefit from further
updates, to make the instructions closer to what's at:

http://rtyley.github.io/bfg-repo-cleaner/#usage

...for instance, using the '--mirror' flag when cloning makes upodating
/all/ branches and tags much less prone to human error.