Tell users about main vs master branches

[yuvipanda]

Ref #159

[consideRatio]

I wondered a bit about the ability to become default-branch-name neutral, but it is a bigger PR to do than mitigation now that there is master/main.

This comment clarified it had been considered already, concluding it didn't seem like a quick an easy fix: #159 (comment)

Let's merge and iterate further if someone has a better solution.