Document that only internal groups are supported
Document that only internal groups are supported

Change-Id: I91f05b0903afb2b91b85091e5dd9e73c629a7044

GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers,davido/gerrit-reviewers-plugin,GerritCodeReview/plugins_reviewers,davido/gerrit-reviewers-plugin