update release notes

RELEASE_NOTES.txt

-Release 1.2 (04/18/2018)
+Release 1.3 (05/XX/2017)
+=============================
+
+- Added APIs and CLIs to update experiment attributes and metadata
+  catalog
+  - New commands:
+    * update-experiment
+    * update-experiment-files
+    * get-async-update-status
+
+Release 1.2 (04/18/2017)
 =============================
 
 - Modified scheduling algorithm for DAQs/uploads to simplify status monitoring