Add a note to the end of BFG output reminding users to expire the reflog

Although documented in http://rtyley.github.io/bfg-repo-cleaner/#usage ,
users sometime miss that instruction, eg:

https://github.com/rtyley/bfg-repo-cleaner/issues/62

Note that '&&' works on Windows as well as BASH:

http://stackoverflow.com/a/8055430/438886

See also https://github.com/rtyley/bfg-repo-cleaner/commit/85d392a7 for
some notes on why we need to clear the reflog.