Update readme with push event support

[vitorbal]

Hello! First of all, thank you for your work on this action! :)

After the changes in this PR, this action should now work properly for push events as well. That's because we now fallback to grabbing the sha from github.context.sha, which will work for push events!

I've updated the README to reflect this.

I also noticed that the github workflows were still referencing a master branch, but that branch was renamed to main. I pushed a commit to fix that too. Thank you!

[vitorbal]

@patrickedqvist 👋 just checking in if you think there's anything else I could do to help land this? Thank you!

[patrickedqvist]

@vitorbal Thanks for checking in.

Could you also update the action.yml?

[vitorbal]

@patrickedqvist done!