Add issue template

ISSUE_TEMPLATE.md

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+This repository is for documentation about the Firefox Architectural Change program. It's not a place to raise bugs about Firefox itself.
+
+* If you have found a bug in Firefox please start here: https://bugzilla.mozilla.org/enter_bug.cgi
+* If you want to give broader feedback: https://input.mozilla.org/
+* If you have a website that doesn't work properly in Firefox: https://webcompat.com/