minor #18046 Add README files where missing (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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Add README files where missing

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #17997
| License       | MIT
| Doc PR        |

The bundles as well as the Swiftmailer bridge did not have readme files.

Commits
-------

a1997b9 add readme files where missing

src/Symfony/Bridge/Swiftmailer/README.md

@@ -0,0 +1,13 @@
+Swiftmailer Bridge
+==================
+
+Provides integration for [Swiftmailer](http://swiftmailer.org/) into the
+Symfony web development toolbar.
+
+Resources
+---------
+
+  * [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/Bundle/FrameworkBundle/README.md

@@ -0,0 +1,10 @@
+FrameworkBundle
+===============
+
+Resources
+---------
+
+  * [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/Bundle/SecurityBundle/README.md

@@ -0,0 +1,10 @@
+SecurityBundle
+==============
+
+Resources
+---------
+
+  * [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/Bundle/TwigBundle/README.md

@@ -0,0 +1,10 @@
+TwigBundle
+==========
+
+Resources
+---------
+
+  * [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/Bundle/WebProfilerBundle/README.md

@@ -0,0 +1,10 @@
+WebProfilerBundle
+=================
+
+Resources
+---------
+
+  * [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)