Add note on $SYS on the command line.

man/mosquitto.8.xml

@@ -87,6 +87,11 @@
 		only sent once per client on subscription. All other topics are updated
 		every <option>sys_interval</option> seconds. If
 		<option>sys_interval</option> is 0, then updates are not sent.</para>
+		<para>Note that if you are using a command line client to interact with the
+			$SYS topics and your shell interprets $ as an environment variable,
+			you need to place the topic in single quotes '$SYS/...' or to
+			escape the dollar symbol: \$SYS/... otherwise the $SYS will be
+			treated as an environment variable.</para>
 		<variablelist>
 			<varlistentry>
 				<term><option>$SYS/broker/bytes/received</option></term>