completed documentation

README.md

 ## Description
 
-This role installs, enables and configures the PHP module `<module>`.
+This role installs, enables and configures the PHP module `pdo`. It is the base of all PDO roles.
 
 ## Requirements
 
 ### Variables
 
+* php_mod_pdo_config_file
+  * description:
+    * This option specifies the PDO config file for a database driver.
+  * default: ""
+
+* php_mod_pdo_dns_aliases
+  * description:
+    * This option contains a list of dictionaries with PDO aliases.
+    * Each dictionary has the entries `name` and `value`.
+    * The `name` entry specifies the DNS alias name.
+    * The `value entry specifies the value for the corresponding DNS alias name. 
+  * default: []
+  * example:
+```
+  php_mod_pdo_dns_aliases:
+    - name: mydb1
+      value: "mysql:dbname=testdb1;host=localhost"
+ 
+    - name: mydb2
+      value: "mysql:dbname=testdb2;host=localhost"
+```
 ## Processes
 
 None