Link contributing guide in README

[recursivetree]

While submitting a PR to this repo, I was asked to add Signed-Off-By: <email> before it can be merged. This requirement is not mentioned in the readme of this repository (compared to e.g. https://github.com/k3s-io/k3s/blob/master/README.md#contributing).

I'd suggest to either copy or link the contributing guidelines in this repo.

[brandond]

fwiw, DCO is required for contribution to all CNCF projects. We could add it to the readme though.

[recursivetree]

This might be obvious to long-term k3s and CNCF contributors, but to a first-time contributor that is not involved in other CNCF projects, it is no obvious fact.

On the other hand, I do agree that it is not the end of the world if a link to the contribution policy is missing. I also don't want to complain about the policy itself, that is up to the k3s team and not me. I'm writing this as feedback to hopefully make the process of contributing more smooth for the next person in the future.