[media-queries-level-5]: Docs for where custom MQs ended up

[kevinSuttle]

• please tag the issue title with the spec's shortname, like [css-foo]
• please link to the spec section you're talking about, or at least the spec
  https://drafts.csswg.org/mediaqueries/

https://lists.w3.org/Archives/Public/www-style/2016Jun/0158.html

• RESOLVED: Move custom MQ to level 5 (issue remove .html for auto-generation #9).

Issue # 9

Rossen: Issue # 9
fantasai: This one isn't 100% baked so defer it to level 5.
Florian: I'm okay with it. I think TabAtkins intended to move to
houdini?
TabAtkins: We'll move it where it needs to be. Doesn't matter.
Rossen: Objections to move custom MQ to level 5?

RESOLVED: Move custom MQ to level 5 (issue #9).

Doesn't appear here:
https://github.com/w3c/csswg-drafts/issues?q=is%3Aopen+is%3Aissue+label%3Amediaqueries-5

[frivoal]

It's here now: https://drafts.csswg.org/mediaqueries-5/