Add design document

Signed-off-by: Audrow Nash <[email protected]>

DESIGN.md

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+# DESIGN
+
+## Features
+
+- Get intraday steps from Fitbit
+- Calculate active steps, as defined by ABM
+- Make a goal for the user
+- Check if the users steps have exceeded the goal
+- Show goals / performance over the duration of the study
+- Test if the API keys work
+- Work with multiple Fitbit devices
+- Export the full study for easy processing
+
+## Considerations
+
+- It would be great to make this deterministic, so that I do not need to keep a
+  memory of the events
+- It would be great to have a sort of caching behavior -> maybe saving to a
+  temporary directory