Update Contributing documentation for dev on Nix

Moves the previous updated blurb from the README to the new CONTRIBUTING
file. Also reworks some wording to link to the Flake nix wiki page for
nix users who haven't enable flakes yet.

CONTRIBUTING.md

@@ -63,10 +63,10 @@ _Learn more about setting up a virtual environment [here](https://docs.python.or
 
   - Run the "TagStudio.sh" script and the program should launch! (Make sure that the script is marked as executable if on Linux). Note that launching from the script from outside of a terminal will not launch a terminal window with any debug or crash information. If you wish to see this information, just launch the shell script directly from your terminal with `./TagStudio.sh`.
 
-  - **NixOS** (TagStudio.sh)
+  - **NixOS** (Nix Flake)
     > [!WARNING]
     > Support for NixOS is still a work in progress.
-    - Use the provided `flake.nix` file to create and enter a working environment by running `nix develop`. Then, run the `TagStudio.sh` script.
+    - Use the provided [Flake](https://nixos.wiki/wiki/Flakes) to create and enter a working environment by running `nix develop`. Then, run the program via `python3 tagstudio/tag_studio.py` from the root directory.
 
 - **Any** (No Scripts)