Update Contributing guide

[smlambert]

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)