minor symfony#9683 Added the missing Installation section in LDAP article (javiereguiluz)

This PR was merged into the 4.0 branch.

Discussion
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Added the missing Installation section in LDAP article

This morning a user on Slack suggested to add this because he didn't execute this command and he got an exception instead of the expected result.

Commits
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708c8e3 Added the missing Installation seciton in LDAP article

Author: javiereguiluz

security/ldap.rst

@@ -34,6 +34,16 @@ This means that the following scenarios will work:
 * Loading user information from an LDAP server, while using another
   authentication strategy (token-based pre-authentication, for example).
 
+Installation
+------------
+
+In applications using :doc:`Symfony Flex </setup/flex>`, run this command to
+install the Ldap component before using it:
+
+.. code-block:: terminal
+
+    $ composer require ldap
+
 Ldap Configuration Reference
 ----------------------------