Add CONTRIBUTING.md to all main 2sxc repos

[iJungleboy]

I'm submitting a

[x] docs request

...about

[x] contributing

Current Behavior / Expected Behavior

There are ca. 4 core repos to 2sxc, and people who want to contribute would have a hard time finding out where to start. So it would be good if they all had such a file in the root folder.

The file should probably be very lightweight and point to the official docs.

Your environment

• 2sxc version(s): 18.02