Add CONTRIBUTING.md
Add CONTRIBUTING.md

github evaluates the maturity of a project in part by whether it has
certain things set up: a readme, a license, a contributions document
and a code of conduct.  There's no cost to adding the CONTRIBUTING.md
which the github mirror will pick up - it's the content from a web page
(and points to that page as well).

Change-Id: Idb77d3b822482ddc430cf500eca0a6bae5ccc955
Signed-off-by: Mats Wichmann <7a5e6f27299d2bbd9d04b59d9e8e4f8eab83d8cb@linux.com>

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