README: add reference to viewer (output) plugin

Author: Lukas955

README.rst

@@ -36,6 +36,8 @@ network interface and a port. Multiple instances of these plugins can run concur
 **Output plugins** - store or forward your flows.
 
 - `JSON <src/plugins/output/json>`_ - convert flow records to JSON and send/store them
+- `Viewer <src/plugins/output/viewer>`_ - convert IPFIX into plain text and print
+  it on standard output
 - `dummy <src/plugins/output/dummy>`_ - simple module example
 - `lnfstore <extra_plugins/output/lnfstore>`_ (*) - store all flows in nfdump compatible
   format for long-term preservation