update release notes

RELEASE_NOTES.txt

+Release 1.6 (09/XX/2017)
+=============================
+
+- DM Station GUI fixes/enhancements:
+  - Added timer for automatic refresh of DAQ/upload status screens
+  - Split configuration options for DAQs/uploads
+  - Fixed issue with duplicate proposal users
+  - Used current date as default for experiment start/end dates
+
 Release 1.5 (08/04/2017)
 =============================