Add README for internal data representation

README.md

@@ -1,6 +1,6 @@
 # habit-migrate
 
-Can take an export of nomie habits in json format and convert it to be importable in Loop Habit Tracker.
+Can take an export of [nomie](https://nomie.app/) habits in json format and convert it to be importable in [Loop Habit Tracker](https://loophabits.org/).
 
 Confirmed working for nomie version 5.6.4 and Loop Habit Tracker version 2.0.2 and 2.0.3. 
 Presumably works for other nomie 5.x versions and other Loop 2.x versions as well, 
@@ -32,3 +32,22 @@ In the future there might be an easier road to using this package but that's the
 The package can also be used as a library to load nomie data
 or move data into Loop Habit Tracker.
 
+## Internal representation
+
+Internally, the data gets represented within three concepts: 
+Events, Activities and Trackers.
+
+Events are simple entries or logs of, basically, anything and represent *qualitative* data. 
+At their most basic, they only describe 'something' at a certain point in time.
+For that, they have to contain a time and they may contain prose text (i.e. an arbitrary text string).
+Additionally, an event can contain a list of one or more activities.
+
+Activities are the changes to whatever is tracked *quantitatively*.
+They always belong to an event and thus the moment in time the event took place.
+They might even be the only interesting thing that took place in the event,
+but not necessarily.
+Lastly, they contain a single tracker which they belong to.
+
+Trackers are the meta-data of whatever is being tracked quantitatively through activities.
+They define a name, label, scores, descriptions, reminders and so on.
+All data being imported is transformed into this model and output from it again.