Updated release notes for HttpClient 5.6.4 release

Author: ok2c

RELEASE_NOTES.txt

@@ -1,3 +1,21 @@
+Release 5.6.4
+------------------
+
+This maintenance release fixes SSL parameter application in the async TLS upgrade
+strategy.
+
+
+Change Log
+-------------------
+
+* BearerScheme to reject control characters in bearer token.
+  Contributed by Javid Khan <dxbjavid at gmail.com>
+
+* Corrects application of SSL parameters in the async TLS upgrade method
+  Contributed by Oleg Kalnichevski <olegk at apache.org>
+
+
+
 Release 5.6.3
 ------------------