The desire to automate it shows that you don't feel like it is a good use of your time -> why would it be worth the receivers time to read your e-mail, especially since it's not clear what they gain from it? If they are closely interested in your project they'll come back to your repo and see updates, a new way to contact you, ... that way, if not it is just a nuisance for them.
The receiver hasn't given you the ok to e-mail them, and sending mass unsolicited e-mail is basically the definition of spam. If you want a special way to contact users, have them clearly opt-in to it. Could be an e-mail list, could be a github issue that you ask them to subscribe to, ...