Link to blog post on v0.4.0 release (#314)

A miniscule change to link to the blog post on `v0.4.0` containing
information on the various changes therein.

CHANGELOG.md

@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Breaking change:** Client `InsertMany` and `InsertManyTx` now return number of jobs inserted as `int` instead of `int64`. This change was made to make the type in use a little more idiomatic. [PR #293](https://github.com/riverqueue/river/pull/293).
 - **Breaking change:** `river.JobState*` type aliases have been removed. All job state constants should be accessed through `rivertype.JobState*` instead. [PR #300](https://github.com/riverqueue/river/pull/300).
 
+See also the [0.4.0 release blog post](https://riverqueue.com/blog/a-few-breaking-changes) with code samples and rationale behind various changes.
+
 ## [0.3.0] - 2024-04-15
 
 ### Added