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

nix-bindings-expr-sys/Cargo.toml

@@ -4,6 +4,8 @@ version = "0.2.0"
 edition = "2021"
 build = "build.rs"
 license = "LGPL-2.1"
+repository = "https://github.com/nixops4/nix-bindings-rust"
+readme = "README.md"
 
 [lib]
 path = "src/lib.rs"

nix-bindings-expr-sys/README.md

@@ -3,3 +3,7 @@
 This crate contains generated bindings for the Nix C API (`nix-expr-c`).
 **You should not have to use this crate directly,** and so you should probably not add it to your dependencies.
 Instead, use the `nix-bindings-expr` crate, which _should_ be sufficient.
+
+## Changelog
+
+See the [nix-bindings-rust changelog](https://github.com/nixops4/nix-bindings-rust/blob/main/CHANGELOG.md).