add --ipv4_only flag to spectatord docs (#188)

docs/spectator/agent/usage.md

@@ -33,6 +33,8 @@ spectatord: A daemon that listens for metrics and reports them to Atlas.
     --enable_socket (Enable UNIX domain socket support. Default is true on Linux
       and false on MacOS.); default: true;
     --enable_statsd (Enable statsd support.); default: false;
+    --ipv4_only (Enable IPv4-only UDP listeners. This option should only be used
+      in environments where it is impossible to run IPv6.); default: false;
     --metatron_dir (Path to the Metatron certificates, which are used for
       external publishing. A number of well-known directories are searched by
       default. This option is only necessary if your certificates are in an