Add summary for primitives namespace

Author: Nick-Stanton

xml/ns-Microsoft.Extensions.Primitives.xml

@@ -1,6 +1,6 @@
 <Namespace Name="Microsoft.Extensions.Primitives">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides primitive types for Microsoft's extension libraries.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>