adding release notes

@lbondaryk @ysahnpark and @mlippert.  This is a pull request purely for the purpose of triggering notification emails to people if anything is added to the Rel_0.90 branch.  This branch should probably not be ever merged into master but we may in certain cases want to merge master into this branch, or cherry-pick commits from master into this branch.

The only thing in this branch that is not in master is a /ReleaseNotes.md file.  We need at least one difference between this branch and master in order to create a pull request.  A release notes files might be handy anyway.