add changelog item

Author: mgrunberg

CHANGELOG.md

@@ -16,6 +16,7 @@
 - [#892](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/892) Add support for if_exists on remove_column
 - [#883](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/885) Fix quoting of ActiveRecord::Relation::QueryAttribute and ActiveModel::Attributes
 - [#893](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/893) Add Active Record Marshal forward compatibility tests
+- [#903](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/903) Raise ActiveRecord::ConnectionNotEstablished on calls to execute with a disconnected connection
 
 #### Changed