added documentation for installing datum grids (#399)

Author: snowman2

docs/installation.rst

@@ -29,6 +29,25 @@ If these installation methods do not meet your needs, the section below provides
 for getting setup.
 
 
+Datum Grids
+===========
+
+You can add datum grids to your PROJ data directory for better accuracy for some transformations.
+
+Available methods for download include:
+
+1. Download stable from https://download.osgeo.org/proj or latest from https://github.com/OSGeo/proj-datumgrid
+
+2. Use `conda <https://conda.io/en/latest/>`__ with the `conda-forge <https://conda-forge.org/>`__ channel:
+
+  You can install datumgrids with:
+
+  .. code-block:: bash
+
+     conda install -c conda-forge proj-datumgrid-europe proj-datumgrid-north-america proj-datumgrid-oceania proj-datumgrid-world
+
+
+
 Installing from source
 ======================