README clarifications

Be more explicit about required PHP version and ways of contributing.
Fix project description

The current project description in the composer.json file doesn't make
any sense.

Update project to something more understandable.
Close connection when the PDO adapter is destroyed

Open database connections will hang around until they are closed due to
the database time out.

Instead of waiting for the database connection to time out causing
degraded performances, close the database connection as soon as object
is destroyed either by developer either by PHP garbage collector.

As per PHP manual it is enough to set PDO instance to 'null'.

Reference: http://php.net/manual/en/pdo.connections.php