Extend CONTRIBUTING.md to describe how to try changes

[ensonic]

I would expect something along the lines of:

in kind:
...

in full fledged cluster:

cd <src checkout>
REGISTRY=gcr.io/<my-gcr>
docker build --network=host --tag ${REGISTRY}/spicedb-operator:latest .
docker push ${REGISTRY}/spicedb-operator:latest
kubectl -n spicedb-operator set image deployment/spicedb-operator spicedb-operator=${REGISTRY}/spicedb-operator:latest
kubectl -n spicedb-operator delete pod -l app=spicedb-operator