[whatwg/streams] Participation agreement requirement non-obvious (#942)

In #941, we got a PR from someone who wasn't aware of the participation agreement requirement. Looking at CONTRIBUTING.md and README.md, nothing stood out as mentioning the requirement.

To avoid wasting people's time, they should be able to see that a participation agreement is required before they start working on a PR.

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/942