Add CONTRIBUTING.md.
Add CONTRIBUTING.md.

Signed-off-by: ThomasRyd <5f50a84c1fa3bcff146405017f36aec1a10a9e38@mender.io>
Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

ThomasRyd/mender,ThomasRyd/mender,bboozzoo/mender,bboozzoo/mender