Add calcualation for locally cloned repositories

[mathias-nyman]

I don't know if this is something that's needed generally, but I'm leaving this here for posterity. At least a one-liner documenting the calculation logic would be convenient if found in the README.md. For example like this, which calculates the amount of contributors in all repositories under and including the current working directory:
find . -name ".git" | xargs -I {} git --git-dir {} log --since="90 days ago" --pretty=format:%ae | sort | uniq | wc -l

With this oneliner for example, one can clearly see that you only calculate contributors on the default branch, not all branches.

But a caveat would be git submodules; in your current calculation logic you do not calculate code contributors in submodules within a repository, although that oneliner would do so..