update release notes

RELEASE_NOTES.txt

+Release 2.3.0 (04/08/2019)
+=============================
+- DM Station GUI fixes/enhancements:
+  - Added support for single file upload
+  - Added support for experiment root path
+  - Added warning dialog for experiment updates
+  - Added number of tooltips 
+
 Release 2.2.2 (02/11/2019)
 =============================
 - Updated deployment scripts