Update release_notes.md for 5.0.2

Author: stanleysmall-microsoft

docs/release_notes.md

@@ -6,6 +6,10 @@ Below is a summary of the the main features/bug fixes in the most recent release
 
 ## Session State Provider Release Notes
 
+### v5.0.2
+
+This version updates Stack Exchange Redis and adds a null check for the serialization methods. 
+
 ### v5.0.1
 
 Removed try-catch around serialization so it now throws an error. Updated the assembly versions to match the release versions. Downgraded Microsoft.Bcl.AsyncInterfaces to version 5.0.0