Merges release 18.2.2 documentation changes

[paulpilone]

Summary: Summary of changes

Addresses CUMULUS-3726: Release 18.2.2.

Changes

• Merges down updated CHANGELOG, CONTRIBUTOR, and aws-client README as a result of releasing 18.2.2

PR Checklist

• [x] Update CHANGELOG
• [ ] Unit tests
• [ ] Ad-hoc testing - Deploy changes and test manually
• [ ] Integration tests