Improve error message when action bubbles
Document available features.

Add `FEATURES.md` as a place for contributors to indicate what each
feature flag does and where we can find out more information about it.
Merge pull request #3583 from alexspeller/improve-action-error-message

Improve error message when action bubbles
Merge pull request #3584 from rjackson/make_features_listing

Document available features.