Add documentation section about custom handlers

[lorenabalan]

Feels like a very underrated feature of mangum that deserves it's moment in the sun. ☀️ 🙃 Following up on https://github.com/jordaneremieff/mangum/issues/288#issuecomment-1495882931

There might be some implementation details I didn't get quite right, particularly in the scope property, so do let me know what's the best way / convention to advertise!

Also updated contribution guide while I'm here, as GitHub dropped support for git protocol.