add rad_development_python to environment and add update instructions

README.md

@@ -20,8 +20,19 @@ For use in other nix-based projects, `flpudt` is available as
 `packages.${system}.flupdt`. Please see our `examples/` folder for common
 use-cases.
 
-## Why the name?
+## Why the name
 
 `flupdt` comes from Fl(ake) Up(date) D(iff) T(ool). The cli is also available as
 `flake-update-diff-tool`, for ease-of-use and those who use screen readers or
 similar accessibility tools that may not react well to `flupdt`.
+
+## Contributing
+
+### Update procedure
+
+Until `rad_development_python` (`rdp`) is in PyPI, we use the below steps to
+update the lock file when there is an update to `rdp`.
+
+1. Run `poetry lock` to update the poetry reference to GitHub
+1. Run `poetry2nix lock` to update the overlay which Poetry2Nix uses
+1. Run `nix flake check` to verify that all is building as expected