Add instruction to expire the reflog

Conversation with @kini reminded me I should be telling people to
do this...

https://twitter.com/rtyley/status/381310179708399616

"git gc tries very hard to be safe about the garbage it collects. In particular, it will keep not only objects referenced by your current set of branches and tags, but also objects referenced by the index, remote-tracking branches, refs saved by git filter-branch in refs/original/, or **reflogs** (which may reference commits in branches that were later amended or rewound)."

-- http://git-scm.com/docs/git-gc

...so it's safest to use "git reflog expire --expire=now --all".