simplify information about shapefiles

"Additionally you need some shapefiles." reworded and moved to correct directory
explain why shapefiles are needed
explain what script is doing once
new version should be shorter and give all info of the current version

INSTALL.md

@@ -26,16 +26,16 @@ Custom indexes are not required, but will speed up rendering, particularly for f
 psql -d gis -f indexes.sql
 ```
 
-Additionally you need some shapefiles.
-
 ## Scripted download
-To download the shapefiles you can run the following script. No further steps should be needed as the data has been processed and placed in the requisite directories.
+Some features are rendered using preprocessed shapefiles.
+
+To obtain them you can run the following script.
 
 ```
 scripts/get-shapefiles.py
 ```
 
-This script generates and populates the *data* directory with all needed shapefiles, including indexing them through *shapeindex*.
+This script downloads necessary files, generates and populates the *data* directory with all needed shapefiles, including indexing them through *shapeindex*.
 
 ## Manual download