Add CHANGELOG and crate READMEs

- Add CHANGELOG.md documenting 0.1.0 and 0.2.0 releases with contributor
  credits and PR links
- Add README.md to each crate linking to the changelog
- Link READMEs in Cargo.toml files
- Add repository field to -sys crates that were missing it
- Update top-level README to reference changelog and reflect -sys crate split
- Document changelog update step in release process

doc/maintainers/release.md

@@ -1,9 +1,18 @@
-
 # Release process
 
 This project uses simple tags, that trigger a release of all crates using Hercules CI.
 See [HCI Effects cargo publish workflow].
 
+## Before tagging
+
+- Update `CHANGELOG.md`:
+  - Make sure the Unreleased section is up to date
+  - Change it to the new version and release date
+
+## After tagging
+
+- Add a new Unreleased section to `CHANGELOG.md`
+
 Dissatisfied with the coarse grained release process? Complain to @roberth and he'll get it done for you.
 
 [HCI Effects cargo publish workflow]: https://docs.hercules-ci.com/hercules-ci-effects/reference/flake-parts/cargo-publish/#_releasing_a_version