Explain how to specify X509 certificate to use for git signing

[schlomo]

I have multiple certs with the same corporate email and I had to manually specify the correct cert to use.

https://docs.github.com/en/free-pro-team@latest/github/authenticating-to-github/telling-git-about-your-signing-key#telling-git-about-your-x509-key explains how, would be nice to mirror/link that info here too.