More release notes updates. Inching closer to iperf-3.2.

RELEASE_NOTES

@@ -67,9 +67,15 @@
     is primarily a cosmetic change to prevent these fairly meaningless
     intervals from showing up in the output (#278).
 
+  * Compatiblity note: Users running iperf3 3.2 or newer from the
+    bwctl utility will need to obtain version 1.6.3 or newer of bwctl.
+    Note that bwctl, a component of the perfSONAR toolkit, has been
+    deprecated in favor of pScheduler since the release of perfSONAR
+    4.0.
+
 * Developer-visible changes
 
-  * Various warnings and build fixes (#551, #564, #518).
+  * Various warnings and build fixes (#551, #564, #518, #597).
 
   * Some improvements have been made for increased compatibility on
     IRIX (#368) and with C++ (#587).