Document `webhook-forwarder.github.com` api

[dhirschfeld]

As mentioned in https://github.com/orgs/community/discussions/38262#discussioncomment-5959296, it would be great if this api were documented so that others could make use of it directly

[gr2m]

I implemented a relay in JavaScript that you could use as a reference for the time being:
https://github.com/gr2m/github-webhook-relay/

[dhirschfeld]

Awesome, thanks for sharing @gr2m!