Provide Better Guidance in CONTRIBUTING.md

[NimJay]

Describe request or inquiry

• We receive a lot of incredible external contributions to this repo (from non-Googlers and Googlers outside the team that owns this repo).
• But our current CONTRIBUTING.md file doesn't contain useful information about how to contribute and what to expect when creating a pull-request.
• To respect external contributors, we should elaborate on these points:
  • some changes need to be tested on a GKE cluster manually (e.g., sometimes using skaffold to make sure development still works)
  • large changes require Google-internal design reviews (i.e., a design phase)
  • each PR will be deployed to a GKE cluster (so some automated testing exists)
  • some product showcases are out-of-scope for this repo and should live in externally-owned repos
  • etc.

Additional Info

• A few examples of awesome external contributions:
  • CI/CD for Multi-Cluster with Istio #796 - Large change that requires Google-internal review and a design phase.
  • microservices-demo in Cloud Run #652 - Large change that is likely out-of-scope for this repo's purpose.
  • Use AR instead of GCR for whereami kubernetes-engine-samples#259 - Small change that was merged.
  • log4j 2nd update #661 - Critical CVE patch.

[NimJay]

Update
There is now a Google Doc for Googlers (go/online-boutique-contribute), but go/online-boutique-contribute does not help non-Googlers (inaccessible) and doesn't contain guidance around pull-request, so let's keep this issue open.
We will see how this issue fits into the 2023 plan for Online Boutique.