Start release notes for iperf-3.6.

While here, get rid of an empty section for 3.5.

RELEASE_NOTES

@@ -1,3 +1,26 @@
+== iperf 3.6 2018-06-?? ==
+
+* Notable user-visible changes
+
+ * A new --extra-data option can be used to fill in a user-defined
+   string field that appears in JSON output.  (#600)
+
+ * A new --repeating-payload option makes iperf3 use a payload pattern
+   similar to that used by iperf2, which could help in recreating
+   results that might be affected by payload entropy (for example,
+   compression).  (#726)
+
+ * -B now works properly with SCTP tests.  (#678 / #715)
+
+ * A compile fix for Solaris 10 was added.  (#711)
+
+* Notable developer-visible changes
+
+ * Functions related to authenticated iperf3 connections have been
+   exposed via libiperf.  (#712 / #713)
+
+ * The ToS byte is now exposed in the libiperf API. (#719)
+
 == iperf 3.5 2018-03-02 ==
 
 * Notable user-visible changes
@@ -10,8 +33,6 @@
     paths.  Many thanks to @FuzzyStatic for providing access to a test
     environment for diagnosing this issue (#692).
 
-* Notable developer-visible changes
-
 == iperf 3.4 2018-02-14 ==
 
 * Notable user-visible changes