Add release notes

Author: Nick Craver

docs/index.md

@@ -27,6 +27,7 @@ Note: to get the latest pre-release build, add ` -Pre` to the end of the command
 - Parameters can now be re-used on subsequent commands (#952 via jamescrowley)
 - Array query support (`.Query<int[]>`) on supported platforms (e.g. Postgres) (#1598 via DarkWanderer)
 - `SqlMapper.HasTypeHandler` is made public for consumers (#1405 via brendangooden)
+- Improves multi-mapping error message when a specified column in splitOn can't be found (#1664 by NickCraver)
 
 ### 2.0.90