Update CHANGELOG

Author: wpolicarpo

CHANGELOG.md

@@ -15,6 +15,7 @@
 - [#763](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/763) Refactor columns introspection query to make it faster
 - [#783](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/783) Update test matrix
 - [#820](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/820) Enable frozen strings for tests
+- [#821](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/821) Enable frozen strings - part 1
 
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