r.sunhours: add link for SOLPOS algorithm

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@57551 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.sunhours/r.sunhours.html

@@ -35,6 +35,18 @@ Calculate photoperiod of day of year 1 (1st January) 2012
 r.sunhours -s sunhour=photoperiod_doy_001 year=2012 day=1
 </pre></div>
 
+<h2>Acknowledgements</h2>
+Acknowledgements: National Renewable Energy Laboratory for their
+<a href="http://rredc.nrel.gov/solar/codesandalgorithms/solpos/">SOLPOS 2.0</a> sun position
+algorithm.
+
+<h2>SEE ALSO</h2>
+<em>
+<a href="g.region.html">g.region</a>,
+<a href="r.sun.html">r.sun</a>,
+<a href="r.sunmask.html">r.sunmask</a>,
+</em>
+
 <h2>AUTHOR</h2>
 
 Markus Metz