Add accumulated release notes from prior iperf-mt public beta releases.
diff --git a/RELNOTES.md b/RELNOTES.md
index 48130e9..94ede52 100644
--- a/RELNOTES.md
+++ b/RELNOTES.md
@@ -1,6 +1,38 @@
iperf3 Release Notes
====================
+iperf-3.15-mt-beta1
+-------------------
+Accumulated release notes from iperf-3.14 and earlier multithreaded
+beta releases:
+
+* Notable user-visible changes
+
+ * Multiple test streams started with -P/--parallel will now be
+ serviced by different threads. This allows iperf3 to take
+ advantage of multiple CPU cores on modern processors.
+
+ * Remove some busy-waiting left over from the original
+ single-threaded implementation, which caused the multi-threaded
+ iperf3 to consume CPU resources for no particular reason, and
+ possible subsequent packet loss.
+
+ * CentOS 7's default compiler is a version of GCC that is too old to
+ compile code using C11 atomic variables. A workaround has been
+ devised for 64-bit CentOS 7 systems, it is not clear whether this
+ approach will work on 32-bit CentOS 7 hosts, or other
+ similarly-vintage build environment.
+
+ * Fix a bug related to idle timeouts, so that the --rcv-timeout
+ option works correctly.
+
+ * Make shutdown of threads more tolerant in the face of various
+ orders of operations at the end of tests.
+
+* Developer-visible changes
+
+ * iperf3 requires pthreads and C atomic variables to compile and run.
+
iperf-3.15 2023-09-14
---------------------
