minor symfony#11489 Update readme for contributing (noniagriconomie)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#11489).

Discussion
----------

Update readme for contributing

Put the paragraph before the note

Commits
-------

21ccd6a Update readme for contributing

Author: javiereguiluz

README.markdown

@@ -6,15 +6,15 @@ This documentation is rendered online at https://symfony.com/doc/current/
 Contributing
 ------------
 
+We love contributors! For more information on how you can contribute to the
+Symfony documentation, please read
+[Contributing to the Documentation](https://symfony.com/doc/current/contributing/documentation/overview.html)
+
 > **Note**
 > Unless you're documenting a feature that was introduced *after* Symfony 3.4
 > (e.g. in Symfony 4.2), all pull requests must be based off of the **3.4** branch,
 > **not** the master or older branches.
 
-We love contributors! For more information on how you can contribute to the
-Symfony documentation, please read
-[Contributing to the Documentation](https://symfony.com/doc/current/contributing/documentation/overview.html)
-
 SymfonyCloud
 ------------