update release notes

RELEASE_NOTES.txt

-Release 2.3.0 (04/08/2019)
+Release 2.3.0 (04/15/2019)
 =============================
 - Updated CDB plugin for the latest CDB API changes
+- Expanded support for HTML-formatted output
 - DM Station GUI fixes/enhancements:
   - Added support for single file upload
   - Added support for experiment root path