Begin working on Habit Screens project

Created a small MVP for digitizing my weekly habits. Much more to come.

Lots of things happening:

- Copied the boilerplate to get started
- Added a brief project-level README
- Outlined my ambitions in design.md

See README and design.md for more context on this project.

scratch/habit-screens/client/shell.nix

@@ -0,0 +1,10 @@
+let
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
+in pkgs.mkShell {
+  buildInputs = with pkgs.elmPackages; [
+    elm
+    elm-format
+    elm-live
+  ];
+}