Add example usage for removing unwanted '.git' paths

An effort to help users who have `.git` folders in their Git repos
(often due to migrating from Mercurial):

https://github.com/rtyley/bfg-repo-cleaner/issues/60#issuecomment-65919279
https://github.com/rtyley/bfg-repo-cleaner/issues/28#issuecomment-29587223

Some related StackOverflow questions from people afflicted by this
problem:

http://stackoverflow.com/questions/16821649/removing-git-directory-from-git-repo
http://stackoverflow.com/questions/22922187/git-remote-error-object-contains-git
http://stackoverflow.com/questions/20289896/migrating-from-kiln-to-github-git-errors-out-on-having-git-folder-in-history?
http://stackoverflow.com/questions/14728751/pushing-a-git-repo-fails-with-error-contains-git