[charter-html] Indicate relationship with the WHATWG (#112)

Given the overlap in scope, and the fact that various specs have shared history, The WHATWG should be at least mentioned in the Liaison / External Group section, including an explanation why some specs that were started in one group were continued / forked in the other, what could cause this to happen again, and how we plan to coordinate.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/charter-html/issues/112