Add CHANGELOG entry.

Author: James McCarthy

CHANGELOG.md

@@ -3,6 +3,7 @@
 #### Features
 
 * [#264](https://github.com/ruby-grape/grape-entity/pull/264): Adds Rubocop config and todo list - [@james2m](https://github.com/james2m).
+* [#265](https://github.com/ruby-grape/grape-entity/pull/265): Adds ability to provide a proc to as: - [@james2m](https://github.com/james2m).
 * [#255](https://github.com/ruby-grape/grape-entity/pull/255): Adds code coverage w/ coveralls - [@LeFnord](https://github.com/LeFnord).
 
 * Your contribution here.