label-when-approved-action

Added by @potiuk

Removing label when you lost or never had an approval status
might not be what you desire to do. In some workflows when
the label is manually set, removing it is automatically is
harmful and it's better to leave the decision about removal
to the commiters.

Adding the parameter solves the problem.

Thanks @potiuk 👏 🚀

• turns missing pr in workflow_run into warning.

Sometimes (quite often really) when PR gets approved, the PR
gets merged rather quickly, without waiting for result of this
action. Or a new PR gets pushed quickly. In those cases PR will
not be found. But this is usually not a problem then and rather
than failing, we should simply print a warning and exit.

• add support for "workflow_run" workflows to avoid lack of permission on PRs from forks

• fix name of the state
• update README

• optional comment to the PR when became approved (requires label to be set)
• updated README