add Contributing.md

CONTRIBUTING.md

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+Speedrun-statechannels repository has an unusual configuration because the end product is intentionally broken: ie, it has specific gaps in implementation that are meant to be filled in by students doing the tutorial.
+
+Making improvements to this implementation-with-holes is... not fun. So the intent here is to maintain side-by-side branches:
+
+- `main`: the student-facing default branch with implementation gaps
+- `completed`: a branch with the gaps filled in
+
+Work to improve the end-result application should be done against the `completed` branch, merged into the completed branch, and then cherry-picked back to `main`.