update release notes

RELEASE_NOTES.txt

@@ -8,6 +8,14 @@ Release 2.2.0 (01/31/2019)
 - Introduced support for API pip and conda packages
 - Added support for a single file upload via the --file-path option to
   upload commands
+- Added support for arbitrary experiment path under the ${DM_STATION_NAME}
+  directory via the --root-path option to the following commands:
+    * add-experiment
+    * update-experiment
+    * ${DM_STATION_NAME}-daq
+    * ${DM_STATION_NAME}-upload
+  Note that if this option is not used, experiment will be located under
+  the experiment type folder, as before. 
 
 Release 2.1.0 (12/31/2018)
 =============================