Document how to become an OpenJS delegate/representative

[ljharb]

The current process for becoming, eg, an Ecma delegate on behalf of OpenJS, is basically:

1. show up to meetings regularly
2. file an issue asking to be one, describing what you'd like to achieve
3. profit!

Nobody's bothered to document this because in over half a decade only like 5-6 people (outside of regular attendees) have expressed interest, but let's document it!