doc: add CHANGELOG.md

I decided to do this because there have been important bug fixes since
the first release, and I'm planning to make the next release.

Which crate's version should it represent? I'm not sure yet. Ideally,
there should be a separate changelog for each API surface (e.g.,
kernel-app, port-kernel). I'll probably first have to put the kernel API
and the kernel implementation in two separate crates somehow.

It would suck if middlewares had to be upgraded every time the kernel
broke the port-to-kernel API but not the app-to-kernel API.

Author: yvt

CHANGELOG.md

@@ -0,0 +1,19 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Fixed
+
+- Fixed: Wrap const generic arguments in braces, fixing builds on the latest compiler version
+- Fixed: Remove `#[naked]` when inlining is prerequisite for correctness; functions with `#[naked]` are no longer eligible for inlining as of [rust-lang/rust#79192](https://github.com/rust-lang/rust/pull/79192).
+
+## 0.1.0 - 2020-11-03
+
+- Initial release.
+
+[Unreleased]: https://github.com/yvt/r3/compare/0.1.0...HEAD