Add check script to the repo

As a contributor, one thing you can do is check whether brew-desc is
missing new formulas or still includes deleted formulas.

The included script `check.sh` does both. It requires git and awk. It's
also written for the `dash` shell, but runs fine via `bash check.sh`.

The script clones the homebrew repo and then builds two lists, done.txt and
homebrew.txt. By diffing the two lists, the script shows what new items
aren't in brew-desc and what removed items still are.

If you're not interested in helping to develop brew-desc, just ignore the
file.