Add a "zuul: gated" status badge

Other CI systems have 'badges' that can be put into web locations
related to the repo. They show various quality marks, such as "build:
passing".

In Zuul, however, we offer a different option - that your build never
stop working.

It would be nice to let people add a "gated" badge to their repo similar
to how other systems have status badges. This is a badge that just says
"zuul: gated" in zuul's blue.

Co-Authored-By: Tobias Henkel <tobias.henkel@bmw.de>
Change-Id: I1af8152c2b471ffe45ea35a819229ca8e41bbc43

www/gated.svg

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