Add user documentation for @wpt-pr-bot

[zcorpan]

Related: #82

There's no documentation about what @wpt-pr-bot does and how to use it.

The wpt docs mention META.yml in a few places, but doesn't mention @wpt-pr-bot specifically.

https://web-platform-tests.org/index.html?highlight=meta%20yml#github
https://web-platform-tests.org/test-suite-design.html?highlight=meta%20yml#test-layout
https://web-platform-tests.org/reviewing-tests/index.html?highlight=meta%20yml#notifications

For #81 and #33 there would be commands, which should be documented somewhere.

Having this documentation in web-platform-tests.org makes it a bit more discoverable I think. The README.md in this repo can then link to that.

[foolip]

If we do add docs, web-platform-tests/wpt#18943 would still make it hard to discover.