Add 'Data Design'

Data-Design.md

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+Design for the data in recklist
+
+## Requirements
+
+- Multiple named checklists
+- Arbitrary number of tasks
+- Arbitrary number of nested tasks
+
+## Data Structures
+
+### Main Data List
+
+- Vector: checklists: can be an arbitrary length, vectors can be appended to and deleted from
+
+### checklist
+
+- String: name: used to name the list itself
+- Vector: list_item: used to hold the actual todo, if it's done, and sub-lists
+
+### todos
+
+- String: todo: used to hold the actual todo item
+- Boolean: done: is done
+- Date: due_by: due date
+- Date: completed_on: date it was checked off
+- Vector: todos: sublist of todos
+
+## Saving
+
+Rust has a way to serialize and dump data structures, still need to figure out the limitations, since these data structures use nested lists and structs.