Updated Hue documentation with ROOT's path

Added the following 2 lines of code under the Build once section (content/developer/development):
#export ROOT which should point to your Hue directory
export ROOT=<path_to_hue_directory>

docs/docs-site/content/developer/development/_index.md

@@ -27,6 +27,9 @@ Build once:
     # Mac user might need to set
     export SKIP_PYTHONDEV_CHECK=true
 
+    # Export ROOT which should point to your Hue directory
+    export ROOT=<path_to_hue_directory>
+
     make apps
 
 The [dependencies documentation](/administrator/installation/dependencies/) is here to help for troubleshooting build issues.