Affinity: Add changelog entries relating to PR #70 discussion

Author: Mark Hoemmen

affinity/cpp-20/d0796r3.md

@@ -16,6 +16,9 @@
 
 ### P0796r3 (SAN 2018)
 
+* Remove reference counting requirement from `execution_resource`.
+* Change lifetime model of `execution_resource`: it now either consistently identifies some underlying resource, or is invalid; context creation rejects an invalid resource.
+
 ### P0796r2 (RAP 2018)
 
 * Introduce a free function for retrieving the execution resource underlying the current thread of execution.