minor #18045 [2.8] update readme files for new components (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
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[2.8] update readme files for new components

| Q             | A
| ------------- | ---
| Branch        | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #17997
| License       | MIT
| Doc PR        |

This completes @javiereguiluz's great work from #17997 by adapting the readme files of components added in Symfony 2.8.

Commits
-------

897ca7f [2.8] update readme files for new components

src/Symfony/Component/Ldap/README.md

@@ -18,10 +18,7 @@ The documentation for the component can be found [online] [0].
 Resources
 ---------
 
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/Ldap/
-    $ composer install
-    $ phpunit
-
-[0]: https://symfony.com/doc/2.8/components/ldap.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)

src/Symfony/Component/PropertyInfo/README.md

@@ -7,8 +7,7 @@ of popular sources.
 Resources
 ---------
 
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/PropertyInfo/
-    $ composer install
-    $ phpunit
+  * [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)