[whatwg/streams] Add Contributing.md with various editing guidelines (#439)

Closes #437; closes #421.

@tyoshino can you think of other things we should include?
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/439

-- Commit Summary --

  * Add Contributing.md with various editing guidelines

-- File Changes --

    A Contributing.md (66)
    M README.md (24)

-- Patch Links --

https://github.com/whatwg/streams/pull/439.patch
https://github.com/whatwg/streams/pull/439.diff

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