FR - Add documentation on how to support multiple email address

[akshetpandey]

It would be great to have documentation on how to associate multiple keys + email id pairs in keybase and github for people using different account for open source and closed source work.

[aaemnnosttv]

Github seems to only recognize the first identity's email when exporting with keybase pgp export -q MYKEY.

I needed to export the public key with gpg --armor --export GPG MYKEY instead, as outlined in the document on GitHub https://help.github.com/articles/associating-an-email-with-your-gpg-key/