feature #5435 Added information about the four sub-components of Secu…

…rity component (javiereguiluz)

This PR was squashed before being merged into the 2.6 branch (closes #5435).

Discussion
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Added information about the four sub-components of Security component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6+
| Fixed tickets | #4151

Commits
-------

839342d Added information about the four sub-components of Security component

components/security/introduction.rst

@@ -21,6 +21,23 @@ You can install the component in 2 different ways:
 
 .. include:: /components/require_autoload.rst.inc
 
+Starting from Symfony 2.4, the Security component is divided into four smaller
+sub-components which can be used separately:
+
+``symfony/security-core``
+    It provides all the common security features, from authentication to
+    authorization and from encoding passwords to loading users.
+
+``symfony/security-http``
+    It integrates the core sub-component with the HTTP protocol to handle HTTP
+    requests and responses.
+
+``symfony/security-csrf``
+    It provides protection against `CSRF attacks`_.
+
+``symfony/security-acl``
+    It provides a fine grained permissions mechanism based on Access Control Lists.
+
 Sections
 --------
 
@@ -30,3 +47,4 @@ Sections
 * :doc:`/components/security/secure_tools`
 
 .. _Packagist: https://packagist.org/packages/symfony/security
+.. _`CSRF attacks`: https://en.wikipedia.org/wiki/Cross-site_request_forgery