Release v0.4.0 & add changelog

We've had enough changes for a version, notably Linux support, so time
for a release!

There is now a changelog where we can jot down changes.

CHANGELOG.md

@@ -0,0 +1,62 @@
+# Changelog
+
+## Unreleased
+
+## [0.4.0](https://github.com/actonlang/acton/releases/tag/v0.4.0) (2021-07-23)
+
+### Added
+- Linux compatibility!
+  - it is now possible to compile the Acton compiler (actonc), the backend and
+    the RTS on Linux
+  - actonc can now compile Acton programs on Linux
+  - Linux on x86_64 is tested, no other architectures
+  - libkqueue is used on Linux as a compatibility shim between kqueue calls and
+    epoll
+    - Acton was initially developed on OS X and the RTS therefore uses kqueue
+      and not epoll / aio(uring)
+- More test cases, in particular the ones for which we have GitHub issues opened
+- CI runs on Linux too
+  - acton is built and tested on both Linux and OS X
+  - all tests are the same across both platforms
+- `actonc` has got a `--version` argument
+
+### Changed
+- Now uses Haskell lts-17.14 instead of lts-13.0
+
+### Removed
+- The project repository has been cleaned up overall
+
+### Fixed
+- `actonc` now detects and reports internal compiler errors
+  - happens when gcc fails to compile the C code that actonc generates
+- A bunch of compiler surprises have been made less surprising
+  - see git log for details ;)
+
+
+## [0.3.0](https://github.com/actonlang/acton/releases/tag/v0.3.0) (2021-04-14)
+
+### Added
+- Acton is now public and published at its new [home on
+  Github](https://github.com/actonlang/acton/)
+- Added basic (working) build instructions
+- A basic test suite with a few tests, including some of the examples being
+  reused for testing
+- GitHub Actions is used as CI to build and test Acton
+  - Mac OS X / Darwin is the only platform used for testing Acton
+
+### Changed
+- distributed database backend is disabled in builds per default
+  - while working, it is too rough around the edges to be enabled per default
+  - in particular, it needs to be made run time configurable
+
+
+## 0.2.0 (a long long time ago, like 2019)
+
+This is a sort of fictitious release that roughly maps to the prehistoric era
+before recorded time. There was a git log but no real version keeping.
+
+0.2.0 was the first ever version of the "new compiler". It wasn't 0.1 because in
+an earlier prehistoric era, another system existed which could be called the
+first incarnation of Acton. It was more of a proof of concept based on Python as
+a runtime and with a hacked up Cassandra database as a backend store. Since
+then, this second incarnation has been in focus and 0.2.0 was its first version.