FLUME-3178. Update RELEASE-NOTES for 1.8.0
FLUME-3178. Update RELEASE-NOTES for 1.8.0

tmgstevens/flume,tmgstevens/flume,szaboferee/flume,tinawenqiao/flume,tinawenqiao/flume,tmgstevens/flume,szaboferee/flume,tinawenqiao/flume,szaboferee/flume,tinawenqiao/flume,tmgstevens/flume,szaboferee/flume