Block direct push to main

[htwangtw]

Yes this is a trouble in paradise.

Allowing it will move things faster, but the number of contributors have increased and having the liberty to push to main can cause potential issues when people work on a fork or a branch, leading to merge conflicts and all that.

To minimise merge conflicts, it's the best to keep the main branch clean. This way both the maintainer and the contributor can stay away from git gymnastics.