README: Link to supported XEPs in wiki

[lnjX]

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[codecov]

Codecov Report

Merging #172 into master will not change coverage.
The diff coverage is n/a.

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  Misses       5556     5556

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[jlaine]

While I'm not absolutely opposed to this, this seem to duplicate the information from:

https://github.com/qxmpp-project/qxmpp/blob/master/doc/xep.doc

Why do we prefer the wiki?

[lnjX]

Oh, I actually wasn't aware of that list, because there's no link to. Then I'd prefer the page in the documentation. Only problems I see: afaik we can't directly link to the html version of the page from the README, because the QXmpp version number is in the URL, currently the page is outdated and it's always from the latest stable release (we could solve that by doing frequent releases :D).

[jlaine]

OK, could you replace this PR by a PR which updates doc/xep.doc for a start? In the future we need to make sure PRs which add support for additional XEPs include the relevant change to doc/xep.doc