CHANGELOG: add description for patch 1556

Author: dannywillems

CHANGELOG.md

@@ -104,6 +104,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   See issue [#1464](https://github.com/o1-labs/mina-rust/issues/1464).
   Fixed in [#1546](https://github.com/o1-labs/mina-rust/pull/1546/)
   ([#1546](https://github.com/o1-labs/mina-rust/pull/1546))
+- **CI**: ensure that `cargo test --doc` works in each crate, fix
+  [#1555](https://github.com/o1-labs/mina-rust/issues/1555), see
+  [#1556](https://github.com/o1-labs/mina-rust/pull/1556)
 
 ## v0.17.0