Document required PHP extensions in php.ini

To people unfamiliar with the code, it is not obvious that
the pdo_* PHP extensions must be enabled.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

INSTALL

@@ -38,6 +38,8 @@ read the instructions below.
         }
     }
 
+    Ensure to enable the pdo_mysql extension in php.ini.
+
 3) Copy conf/config.proto to /etc/aurweb/config and adjust the configuration
    (pay attention to disable_http_login, enable_maintenance and aur_location).
 

TESTING

@@ -13,6 +13,8 @@ INSTALL.
 
     # pacman -S php php-sqlite sqlite
 
+   Ensure to enable the pdo_sqlite extension in php.ini.
+
 3) Prepare the testing database:
 
     $ cd /path/to/aurweb/schema