Proposal: Add tested platforms to documentation

[wimrijnders]

To inspire confidence to potential users, I think it's a good idea to add a list of platforms to the README, to indicate which combinations of platforms have been tested. It would look like this:

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Platforms tested on

Pi Model	Revision number	Distribution	gcc version
Model B Rev 2	000d	your distro	your gcc version
Model B Rev 2	000e	Raspian 9.4 (stretch)	6.3.0
2 Model B Rev 1.1	a01041	Raspian 9.4 (stretch)	6.3.0
3 Model B Rev 1.2	a0282	Raspian 9.4 (stretch)	6.3.0
Intel (emulation only)	---	Suse LEAP 42.3	4.8.5

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On every release to branch master, all of these should be tested, at the very least by running make QPU=1 test on all listed machines. The intel machine will run in emulation mode, of course.

Which brings me to the next point: There should be a release procedure. I.e. a list of steps to do on updating master.