add link to coordinate operations by PROJ library in projectionintro (#297)

doc/projectionintro.html

@@ -60,6 +60,7 @@ A graphical user interface is provided by <a href="wxGUI.html">wxGUI</a>.
 <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://crs.bkg.bund.de/crs-eu/">Information and Service System for European Coordinate Reference Systems - CRS</a>
 </ul>