Document all-branches mode

[iainelder]

#45 introduced a new feature enabled by --all-branches.

I don't see it in the documentation yet. I had to experiment with it to figure out how it works.

Perhaps it would be useful to add simple usage examples or sample output to the README. At least least the --all-branches option should appear there.

$ git -C clustergit --no-pager log --branches --not --remotes --no-walk --decorate --oneline
b09d21c (HEAD -> master) Add pic
3210f8a (otherbranch) Add file
$ clustergit -d .
Scanning sub directories of ['.']
./clustergit                            : No Changes, Unpushed commits
Done
$ clustergit -d . --all-branches
Scanning sub directories of ['.']
./clustergit                            : No Changes, Unpushed commits, Unpushed commits on branches: [otherbranch]
Done