Added first notes for data format.

NOTES.md

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+# Recorded data
+
+Format:
+
+```
+Name - [protobuf type] : [unit]
+```
+
+- Speed - `float` : metres / second
+- Cadence - `float` : metres / second
+- Location:
+    - Latitude - `double` : degrees
+    - Longitude - `double` : degrees
+    - Altitude - `float` : metres above sea level
+- Power - `float` : watts
+- Relative wind speed - `float` : metres / second
+- Heart rate - `float` : beats / minute
+- Body temperature - `float` : temperature in kelvin, can be either skin or core temperature
+- Environment temperature - `float` : temperature in kelvin, can be either air temperature or water temperature
+- Oxygen saturation - `float` : percentage, 0-100%, usually peripheral oxygen saturation (SpO2)
+- Blood glucose - `float` : millimoles of glucose per litre of blood
+- Lactic acid - `float` : millimoles of lactic acid per litre of blood
+- Gradient - `float` : percentage, 0-100%
+- Left-Right power balance - `float` : percentage, 0-100% for the right side (55 would imply 45% for the left side, 55% for the right side)
+
+# Comparison of file types
+
+[GPX](https://www.topografix.com/gpx/1/1/gpx.xsd) - GPS Exchange format
+
+[TCX](https://www8.garmin.com/xmlschemas/TrainingCenterDatabasev2.xsd) - Garmin Training Center format
+
+[FIT](https://developer.garmin.com/fit/protocol/) - Flexible and Interoperable Data Transfer protocol
+
+| File type | Location | Cadence | Heart rate | Power | Wind speed | Speed | Body temperature | Environment temperature | Oxygen saturation | Blood glucose | Lactic acid | Gradient | L-R balance |
+| - | - | - | - | - | - | - | - | - | - | - | - | - | - |
+| GPX | 🟢 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 |
+| TCX | 🟢 | 🟢 | 🟢 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 | 🔴 |
+| FIT | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🔴 | 🔴 | 🔴 | 🟢 | 🟢 |
+| Proculite JSON | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 | 🟢 |