Add release notes: v0.19.0-0.4.0

Add changelog entry / release notes for the upcoming release.

Author: tcharding

CHANGELOG.md

@@ -1,4 +1,24 @@
+## v0.19.0-0.4.0
 
+The major change in this version is the Minimum Supported Rust Version (MSRV) bump, we now support
+an MSRV of 1.41.1, along with this change we moved to using a new version number format:
+<bitcoin-core-version>-<lib-version>. This is valid semantic versioning and implies we are pre-1.0
+still.
+
+You can now use `Error` in a more ergonomic manner because we implemented `std::error::Error`. We
+found and fixed an off by one error in the activation height values we were matching against. We
+moved away from using AppVeyor for continuous integration and now use GitHub actions.
+
+You can check out the following pull requests for more information:
+
+- Bump MSRV and enable edition 2018 [#34](https://github.com/rust-bitcoin/rust-bitcoinconsensus/pull/34)
+- Changed format of version number to <bitcoin-core-version>-<this-lib-version>
+- Fix activation heights [#30](https://github.com/rust-bitcoin/rust-bitcoinconsensus/pull/30)
+- Moved to GitHub actions instead of AppVeyor for CI [#38](https://github.com/rust-bitcoin/rust-bitcoinconsensus/pull/38)
+- Implement `std::error::Error` for `Error` [#45](https://github.com/rust-bitcoin/rust-bitcoinconsensus/pull/45)
+- Documentation improvements [#35](https://github.com/rust-bitcoin/rust-bitcoinconsensus/pull/35/commits)
+
+Enjoy!
 
 ## v0.19.0-2