Create readme.md

Visible/RGB Vegetation Indices contrib module description, usage, and bibliography

contrib/visveg/readme.md

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+# Visible Vegetation Indexes
+
+This script produces a Vegetation Index raster from a RGB orthophoto (odm_orthophoto.tif in your project)
+
+## Requirements
+* rasterio (pip install rasterio)
+* numpy python package (included in ODM build)
+
+## Usage
+```
+vegind.py <orthophoto.tif> index
+
+positional arguments:
+  <orthophoto.tif>  The RGB orthophoto. Must be a GeoTiff.
+  index     Index identifier. Allowed values: ngrdi, tgi, vari
+
+Output will be generated with index suffix in the same directory as input.
+
+## Examples
+
+`python vegind.py /path/to/odm_orthophoto.tif tgi`
+
+Orthophoto photo of Koniaków grass field and forest in QGIS: ![](http://imgur.com/K6x3nB2.jpg)
+The Triangular Greenness Index output in QGIS (with a spectral pseudocolor): ![](http://i.imgur.com/f9TzISU.jpg)
+Visible Atmospheric Resistant Index: ![](http://imgur.com/Y7BHzLs.jpg)
+Normalized green-red difference index: ![](http://imgur.com/v8cmaPS.jpg)
+
+## Bibliography
+
+1. Hunt, E. Raymond, et al. "A Visible Band Index for Remote Sensing Leaf Chlorophyll Content At the Canopy Scale." ITC journal 21(2013): 103-112. doi: 10.1016/j.jag.2012.07.020
+(https://doi.org/10.1016/j.jag.2012.07.020)