Clarify where to get extensions for metadata names

[travisleithead]

Cloned from https://www.w3.org/Bugs/Public/show_bug.cgi?id=14363

Current spec text:

4.2.5.2. Other metadata names

Extensions to the predefined set of metadata names may be registered in the WHATWG Wiki MetaExtensions page. [WHATWGWIKI]

Anyone is free to edit the WHATWG Wiki MetaExtensions page at any time to add a type. These new names must be specified with the following information:
[...]

If we're still happy with the WHATWG wiki being the way to extend the spec, then we can close this. But there seems to be some pushback on whether this is legitimate, especially if tools want to have a way of staying current.

@jribbens

[jribbens]

Indeed, having conformance tools attempt to scrape an arbitrarily-formatted (non-computer-parseable) and fragile (publicly editable) wiki page to try and guess at the list of valid values seems dubious at best.

[chaals]

This is a duplicate of #160