Update Contributing guide

Expectations/notes to add:
- PRs should be related to a descriptive issue (giving context for the change and if possible estimated severity of the problem)
- Between approvals and merging, unless invited by the reviewers to make small changes before merging, PRs should not receive more commits/changes by the author
- If test or framework changes, 2 types of verification expected 1) that local cmd line testing still works 2) running in Jenkins still works (Grinder)