Update RELEASE_NOTES
Update RELEASE_NOTES

Update with changes since 0.20 to prepare for a 0.21 release.

Change-Id: Iaf03108bb3e91eb3431dd660afb22d31c2498613

openstack/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools,savinash47/openstack-doc-tools