Add system command notes.

documentation/RELEASE_NOTES.html

@@ -70,6 +70,9 @@ an mbboRecord it thought the field was a USHORT instead of an ENUM.</p>
 <p>A new function has been added epicsStrPrintEscaped, which converts
 the standard C escape characters to \xxx characters.</p>
 
+<h4>IOC shell system command</h4>
+<p>The 'system' command has been added to the IOC shell.  To enable this command, add <code>registrar(iocshSystemCommand)</code> to an application database description file.</p>
+
 <center>
 <h2>Changes since 3.14.3</h2>
 </center>