improve readme

README.md

@@ -27,11 +27,11 @@ There are also proof-of-concept ports of this script that only support basic RES
 
 ## Requirements
 
-  - PHP 7.0 or higher with PDO drivers for MySQL, PgSQL, SqlSrv or SQLite enabled
-  - MySQL 5.6 / MariaDB 10.0 or higher for spatial features in MySQL
-  - PostGIS 2.0 or higher for spatial features in PostgreSQL 9.1 or higher
-  - SQL Server 2012 or higher (2017 for Linux support)
-  - SQLite 3.16 or higher (spatial features NOT supported)
+  - PHP 7.0 or higher with PDO drivers enabled for one of these database systems:
+    - MySQL 5.6 / MariaDB 10.0 or higher for spatial features in MySQL
+    - PostGIS 2.0 or higher for spatial features in PostgreSQL 9.1 or higher
+    - SQL Server 2012 or higher (2017 for Linux support)
+    - SQLite 3.16 or higher (spatial features NOT supported)
 
 ## Installation