Update README to provide more details on how this action works.

:wave: Hello and thanks for pinging us! You've entered our first responder queue. An accessibility first responder will review this soon.

:computer: On PRs for our review: please provide a review environment with steps to validate, screenshots (with alt text), or videos demonstrating functionality we should be checking. This will help speed up our review and feedback cycle.
:warning: If this is urgent, please visit us in #accessibility on Slack and tag the first responder(s) listed in the channel topic.

github / accessibility-alt-text-bot