Add CHANGELOG.md entry about unsafe fork.
Add CHANGELOG.md entry about unsafe fork.

geofft/nix-rust,geofft/nix-rust,nix-rust/nix,asomers/nix,carllerche/nix-rust,nix-rust/nix,carllerche/nix-rust,geofft/nix-rust,asomers/nix,carllerche/nix-rust