README: Add explanation and references for how the binocular stream is generated.

Author: papr

README.md

@@ -22,3 +22,5 @@ Plugin for _[Pupil Capture](https://github.com/pupil-labs/pupil/wiki/Pupil-Captu
 The plugin opens a single outlet named `pupil_capture` that follows the [Gaze Meta Data](https://github.com/sccn/xdf/wiki/Gaze-Meta-Data) format.
 
 See our [pupil-helpers](https://github.com/pupil-labs/pupil-helpers/tree/master/LabStreamingLayer) for examples on how to record and visualize the published data.
+
+The published LSL data is simply a flattened version (see `extract_*()` functions in `pupil_lsl_relay.py`) of the original Pupil gaze data stream. The stream's channels will be filled with best effort, i.e. if there is a monocular gaze datum the values for the opposite eye will be set to `NaN`. The actual pairing of pupil data to binocular gaze data happens in [Capture](https://github.com/pupil-labs/pupil/blob/master/pupil_src/shared_modules/calibration_routines/gaze_mappers.py#L95-L140) and is not a LSL specific behaviour. Therefore, it is prossible to apply the same [flattening code](https://github.com/papr/App-PupilLabs/blob/master/pupil_lsl_relay.py#L226-L287) to offline calibrated gaze data and reproduce the stream published by the LSL outlet.