add links SISO and DIS/RPR-FOM product study group

specifications/README.md

@@ -5,6 +5,8 @@ real-time platform-level wargaming across multiple host computers and is used wo
 especially by military organizations but also by other agencies such as those involved in
 space exploration and medicine.*
 * Wikipedia: [Distributed Interactive Simulation](https://en.wikipedia.org/wiki/Distributed_Interactive_Simulation)
+* [Simulation Interoperabilty Standards Organization (SISO)](https://www.sisostds.org)
+* SISO Product Support Group (PSG): [DIS / RPR FOM PSG - Distributed Interactive Simulation / Real-time Platform Reference Federation Object Model](https://www.sisostds.org/StandardsActivities/SupportGroups/DISRPRFOMPSG.aspx)
 
 Manual download links follow.  IEEE standards must be downloaded manually while within NPS campus or firewall.