bulk rename to flupdt, add CLI, add README, add .gitignore

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

README.md

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+# Flake Update Diff Tool
+
+This is the Nix flake update validator. A tool which is able to evaluate a flake
+at two points in history, check that everything evaluates, and provide a diff
+of the two. In its target state, it will provide a similar function to
+DeterminateSystems' awesome `update-flake-lock` tool, but with `nvd` integration
+and other bells and whistles that we've come to like.
+
+## How to Use
+
+Currently, this only supports locally-stored flakes, although we are planning to
+add support for `git`-based URLs for usage outside of CI pipelines where the
+repository is already downloaded.
+
+``` shell
+nix run github:RAD-Development/flake-update-diff-tool -- <path to local flake>
+```
+
+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?
+
+`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`.