cheet-sheet.md: Add a Git-specific cheat sheet

cheet-sheet.md: Add a Git-specific cheat sheet

READMEs and COPYING are all well and good, but this project will
probably make more sense if it has some actual content that it's
trying to development.  I'll use the cheat sheet from
http://git.tremily.us/?p=swc-version-control-git.git as an example,
but this would usually be a paper you're writing or code you're using
in your research, or something else you find interesting.

After writing the file in your text editor, staging and committing it
is the usual:

  $ git add cheat-sheet.md
  $ git commit --all --verbose

The long form options (`--all` and `--verbose`) also have short-form
aliases (`-a` and `-v`), which is why the cheat sheet suggests:

  $ git commit -a ...

That is the same as:

  $ git commit --all ...

Because I think this commit gets things into a useful state, I'll tag
it as version 1.0 and release it to the world.  You can tag the last
commit with:

  $ git tag v1.0