Add 3.4.0 release to Supervisor 4 changelog

Author: mnaberez

CHANGES.txt

@@ -37,6 +37,9 @@
   same time.  Specifying a log filename of ``syslog`` is still supported
   but deprecated.  Patch by Jason R. Coombs.
 
+3.4.0 (2019-04-05)
+------------------
+
 - FastCGI programs (``[fcgi-program:x]`` sections) can now be used in
   groups (``[group:x]``).  Patch by Florian Apolloner.