Process Follow-ups

[gkatsev]

Follow-ups:

• Add "Implemented" folder or Stages like TC39 proposals and update the process to include these
  • update proposal to include a link to the implementation PR
• Add https://github.com/muxinc/custom-video-element as the canonical implementation of these proposals, but not the only implementation
• Update the wording in the README to be more general of "Media Elements"
• Clarify how to call a CfC (comment on the PR)

Originally posted by @gkatsev in #5 (comment)