contributing.md: added more pre-req detail to the beginning of the edit
Added information pertaining to nix and git-lfs installation required to work with urbit's interfaces.
@@ -251,9 +251,13 @@ $ git lfs pull
## Interface development
-Designing interfaces within urbit/urbit requires that the above steps for
-fake `~zod` initialization are followed. Once your fake ship is running and you
-see
+[nix](https://github.com/NixOS/nix) and `git-lfs` should be installed at
+this point, and have been used to `make build` the project.
+
+Designing interfaces within urbit/urbit additionally requires that the
+above steps for fake `~zod` initialization have been followed.
+
+Once your fake ship is running and you see
```
~zod:dojo>
```
