[permissions] Update contributing.md (#58)

This is linking to the correct mailing list and update the guidance.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/permissions/pull/58

-- Commit Summary --

  * Update contributing.md

-- File Changes --

    M contributing.md (39)

-- Patch Links --

https://github.com/w3c/permissions/pull/58.patch
https://github.com/w3c/permissions/pull/58.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/pull/58