Replacing 2.0 version by 2.1

as 2.0 version is deprecated, as told by WouterJ in symfony#2595

Author: gregquat

README.markdown

@@ -8,8 +8,8 @@ Contributing
 
 >**Note** 
 >Unless you're documenting a feature that's new to a specific version of Symfony 
->(e.g. Symfony 2.1), all pull requests must be based off of the **2.0** branch, 
->**not** the master or 2.1 branch.
+>(e.g. Symfony 2.2), all pull requests must be based off of the **2.1** branch, 
+>**not** the master or 2.2 branch.
 
 We love contributors! For more information on how you can contribute to the
 Symfony documentation, please read