I guess it depends on what "developer friendly" means. If it means 'easiest method for a developer to contact me' that's email. It's ubiquitous. Everyone has at least one. It's easy.
If it means 'easiest method for only developers to contact me' that would probably be some documented REST API.
Email address and / or GitHub. No one's saying he needs to show off what a great programmer he is through his contact methods, just don't use a site that's infamously bad for dark patterns and recruiter spam.
It does pose the question, what is the most developer friendly contact method??
GET /contacts
POST /message
Interested to hear ideas.