Add notes on The BFG as an alternative to git-filter-branch

The BFG is dramatically faster for removing unwanted data (like big
files or private data) from Git repository history - and it's much
easier to use too, so it's nice to let people know it's available.
Merge branch 'bfg-alternative-to-git-filter-branch' of https://github.com/rtyley/GitFixUm into gh-pages
Tweak recommendation and usage for BFG