docs: vis module

src/pipe/modules/vis/readme.md

@@ -1,11 +1,18 @@
 # vis: visualise input data with colour ramps
 
-this converts the luminance of the linear input signal to an srgb viridis colour map.
+this converts the luminance of the linear input signal to an srgb viridis colour map,
+like in the following example:
+
+[![viridis map](scatter.jpg)](scatter.jpg)
+
+note that the output will be sRGB, so if you need rec2020, insert an instance
+of `colour` after this and set it to `image` mode.
+
 
 ## connectors
 
-* `input`
-* `output`
+* `input` rec2020 values, the luminance will be visualised
+* `output` the sRGB viridis coloured version of the input
 
 ## parameters