feature #6465 Added ldap to the list of user providers (AAstakhov)

This PR was merged into the 2.8 branch.

Discussion
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Added ldap to the list of user providers

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8+
| Fixed tickets | n/a

Ldap is the new user provider in  Symfony 2.8, so it would be good to mention it in the list of all user providers.

Commits
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f723ea5 Added ldap to the list of user providers

Author: xabbuh

cookbook/security/custom_provider.rst

@@ -9,7 +9,8 @@ When a user submits a username and password, the authentication layer asks
 the configured user provider to return a user object for a given username.
 Symfony then checks whether the password of this user is correct and generates
 a security token so the user stays authenticated during the current session.
-Out of the box, Symfony has an "in_memory" and an "entity" user provider.
+Out of the box, Symfony has four user providers: ``in_memory``, ``entity``, 
+``ldap`` and ``chain``.
 In this entry you'll see how you can create your own user provider, which
 could be useful if your users are accessed via a custom database, a file,
 or - as shown in this example - a web service.