docs: add CONTRIBUTING.md at root of repo, directing to docs/ (#3121)

Add a root CONTRIBUTING.md that directs to docs/CONTRIBUTING.md

# Details

- having a `CONTRIBUTING.md` at the root of the repo is a common convention
  - I looked at the root of the repo initially, didn't find it, then checked the website, but potential contributors may think it doesn't exist as a result

Shamelessly copied from argoproj/argo-workflows#10969 (thank you @agilgur5)

Signed-off-by: jmeridth <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1 @@
+See [docs/CONTRIBUTING.md](docs/CONTRIBUTING.md).