Add a changelog entry for the 0.2.1 release

Author: weiznich

CHANGELOG.md

@@ -4,6 +4,13 @@ All user visible changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/), as described
 for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/text/1105-api-evolution.md)
 
+## [0.2.1] - 2023-03-08
+
+* Dependency updates for `mobc` and `mysql-async` to allow newer versions as well 
+* Extend the README
+* Improve the version constraint for diesel so that we do not end up using a newer
+ diesel version that's incompatible
+
 ## [0.2.0] - 2022-12-16
 
 * [#38](https://github.com/weiznich/diesel_async/pull/38) Relax the requirements for borrowed captures in the transaction closure