[cherry-picker] Document config options

[webknjaz]

Now that this tool supports working with other repos I wanted to check how can I use it in other project.

It took me a while to realize that check_sha should contain very fist commit in a repo's main branch. So I suggest documenting the config usage somewhere in README.

[Mariatta]

It is documented under "Available config options" here?

[webknjaz]

Oh, I think I missed that, thanks. But still I'm not sure what is the purpose of it.

[Mariatta]

Just a way to ensure that we are operating on the right repo, before even starting to do git fetch, checkout, etc.
It can be any commit hash in your repo. We picked the first ever commit to CPython, we know it won't be amended or dropped ever.

[Mariatta]

I'm closing your issue since it's been documented.