[editing-explainer] Add link to charter. (#23)

README: add a link to the gh-pages version of the charter.
You can merge this Pull Request by running:

  git pull https://github.com/AFBarstow/editing-explainer patch-3

Or you can view, comment on it, or merge it online at:

  https://github.com/w3c/editing-explainer/pull/23

-- Commit Summary --

  * Add link to charter.

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/w3c/editing-explainer/pull/23.patch
https://github.com/w3c/editing-explainer/pull/23.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/editing-explainer/pull/23