add GRIM reference links

specifications/README.md

@@ -81,3 +81,10 @@ SISO-REF-010 specifies numerical values and associated definitions for fields th
 [Operations Manual (OPMAN)](https://www.sisostds.org/DigitalLibrary.aspx?Command=Core_Download&EntryId=47284)
 
 ---
+
+**6. SISO-STD-001-2015: Standard for Guidance, Rationale, and Interoperability Modalities (GRIM) for the Real-time Platform Reference Federation Object Model (RPR FOM)** <a href="https://www.sisostds.org/ProductsPublications/ReferenceDocuments.aspx" target="_blank"><img src="SisoLogo.jpg" width="314" align="right"/></a>
+* https://www.sisostds.org/ProductsPublications/Standards/SISOStandards.aspx (scroll down near bottom of page)
+SISO-STD-001-2015 encapsulates guidance in the use of the RPR FOM. It provides descriptions of FOM classes and data types and the relationship between Distributed Interactive Simulation (DIS) and the High Level Architecture (HLA)-based RPR FOM, as well as rules for accomplishing specific distributed simulation tasks.
+* Version 2.0 (10 Aug 2015)
+
+---