Add CHANGELOG.md entry about unsafe fork.

Author: vi

CHANGELOG.md

@@ -12,6 +12,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 (#[1284](https://github.com/nix-rust/nix/pull/1284))
 - Changed unistd::{execv,execve,execvp,execvpe,fexecve,execveat} to take both `&[&CStr]` and `&[CString]` as its list argument(s).
   (#[1278](https://github.com/nix-rust/nix/pull/1278))
+- Made `unistd::fork` an unsafe funtion, bringing it in line with [libstd's decision](https://github.com/rust-lang/rust/pull/58059).
+  (#[1293](https://github.com/nix-rust/nix/pull/1293))
 ### Fixed
 ### Removed