Added details for installing Grass version

Clarified that code works only with Grass 7 or higher version

contrib/grass/README.md

@@ -3,7 +3,7 @@
 Generates DEM, contour and textured relief maps.
 
 ## Requirements
-* GRASS
+* Compile and install GRASS 7 version or higher, https://grasswiki.osgeo.org/wiki/Compile_and_Install
 * Environment variables:
   * PYTHONHOME set to the location of Python
   * PYTHONPATH set to the location of GRASS Python libs
@@ -13,4 +13,4 @@ Generates DEM, contour and textured relief maps.
 ## Usage
     python odm_grass.py <project-path>
 
-Output is `<project-path>/odm_georeferencing/odm_dem.tif`, `<project-path>/odm_georeferencing/odm_contour.shp` and `<project-path>/odm_orthophoto/odm_relief.tif`.
+Output is `<project-path>/odm_georeferencing/odm_dem.tif`, `<project-path>/odm_georeferencing/odm_contour.shp` and `<project-path>/odm_orthophoto/odm_relief.tif`.