add link to coordinate operations by PROJ library in r.proj

nice overview about projections, conversions, transformations, pipeline operator available in the new PROJ library generation

raster/r.proj/r.proj.html

@@ -288,6 +288,7 @@ <h2>REFERENCES</h2>
 <ul>
   <li> <a href="https://www.asprs.org/asprs-publications/grids-and-datums">ASPRS Grids and Datum</a>
   <li> <a href="http://geotiff.maptools.org/proj_list/">Projections Transform List</a> (PROJ)
+  <li> <a href="https://proj.org/operations/index.html">Coordinate operations</a> by PROJ (projections, conversions, transformations, pipeline operator) 
   <li> <a href="http://www.mapref.org">MapRef -
       The Collection of Map Projections and Reference Systems for Europe</a> 
   <li> <a href="http://www.crs-geo.eu">Information and Service System for European Coordinate Reference Systems - CRS</a>