Update README.md

Adds description of false colour composite of thumbnails

README.md

@@ -45,7 +45,10 @@ Contains a global coverage of Sentinel-1 (RTC) patches, each of size 1,068 x 1,0
 |--------|---------|------------|
 | VV | Received Linear Power in the VV Polarization | 10m | 
 | VH | Received Linear Power in the VV Polarization | 10m | 
-| thumbnail | Rescaled false colour saved as png | 10m | 
+| thumbnail | Rescaled false colour<sup>1</sup> saved as png | 10m | 
+
+
+<sup>1</sup> False colour composites are made with decibel-scale values with red green and blue defined as ```R:VV  G:VV+VH  B:VH```. For each channel, a contrast-stretch is applied, transforming minimum-maximum to 0-255.  This means bluer areas have relatively higher VH values, whilst brightness is a function of overall intensity. This is relative within each thumbnail because of the normalisation, and so cannot be compared across different samples.
 
 ## Spatial Coverage
 This is a global monotemporal dataset. Nearly every piece of Earth captured by Sentinel-1 is contained at least once in this dataset (and only once, excluding some marginal overlaps). The coverage is about 35% lower than for Core Sentinel-2 dataset due to the sensor coverage limitations.