See https://keybase.io

Signs all of the directory contents (except that
which is excluded in .gitignore) with an OpenPGP
compliant cryptographic signature.

This creates a SIGNED.md file in the root of the
repository. This file also gets added to the git
index prior to commit.

Note, due to the way keybase works, if there are
files or changes in the working dir that are not
being committed they will still be included with
the signature. In that case someone running
'keybase dir verify' will not get a clean verify
since their directory does not match exactly. It
is best to stash away work dir contents prior to
commit if concerned about this.