Add the database abstraction layer in the USAGE

git-svn-id: https://svn.fournier38.fr/svn/ProgSVN/trunk@1583 bf3deb0d-5f1a-0410-827f-c0cc1f45334c

docs/USAGE

@@ -209,7 +209,15 @@ The fields are defined by :
 The CSRF parameter is verify automatically when using $f->values ();
 In case of error, an exception is raised.
 
-10. Authentication / Authorization
+10. Database abstraction
+------------------------
+A database abstraction permits to forget if the database engine is MySQL, SQLite
+or PostgreSQL. It can read, insert, update easily from arrays, but display the
+tables without having to think about the backend. 
+It check the foreign keys before doing insertions/updates, look at unique
+constrains.
+
+11. Authentication / Authorization
 ----------------------------------
 The Authentication/Authorization module permits to allow/deny access to objects.
 The user must be validated before using a information. It is based on Unix