r.terraflow: Add flow direction encoding information to the manual (#…

…2988)

* r.terraflow: Add flow direction encoding information to the manual

* Add undetermined (0) and undefined (1)

raster/r.terraflow/r.terraflow.html

@@ -62,6 +62,18 @@ <h2>DESCRIPTION</h2>
 a sink-less terrain in which every cell has a downslope flow path
 leading outside the terrain and therefore every cell in the terrain
 can be assigned SFD/MFD flow directions as above.
+Flow directions are encoded using powers of two clockwise starting from
+2<sup>0</sup> for east to 2<sup>7</sup> for north-east.
+
+<div align="center">
+<a href="rterraflow_direction_encoding.png">
+<img src="rterraflow_direction_encoding.png" width="200" height="200" border="0" alt="r.terraflow direction encoding">
+</a><br>
+Flow direction encoding clockwise starting from 2<sup>0</sup> for east to
+2<sup>7</sup> for north-east; 0 for undetermined (sinks) and 1 for undefined
+(null cells)
+(<a href="https://idea.isnew.info/how-to-import-arcgis-flow-direction-into-grass-gis.html">source</a>)
+</div>
 
 <p>Once flow directions are computed for every cell in the terrain,
 <em>r.terraflow</em> computes flow accumulation by routing water using