minor #18095 Updated some missing READMEs (javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
----------

Updated some missing READMEs

| Q             | A
| ------------- | ---
| Branch        | 2.8+
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

8cfd6b8 Updated some missing READMEs

src/Symfony/Component/Security/Guard/README.md

@@ -8,15 +8,8 @@ total control.
 Resources
 ---------
 
-Documentation:
-
-https://symfony.com/doc/2.8/book/security.html
-
-Tests
------
-
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/Security/Guard/
-    $ composer.phar install --dev
-    $ phpunit
+  * [Documentation](https://symfony.com/doc/current/components/security/index.html)
+  * [Contributing](https://symfony.com/doc/current/contributing/index.html)
+  * [Report issues](https://github.com/symfony/symfony/issues) and
+    [send Pull Requests](https://github.com/symfony/symfony/pulls)
+    in the [main Symfony repository](https://github.com/symfony/symfony)