Contributions should be based off 2.3 or higher

Per the documentation (http://symfony.com/doc/current/contributing/documentation/overview.html#contributing), the base branch should be 2.3, not 2.2.

Author: colinodell

README.markdown

@@ -7,9 +7,9 @@ Contributing
 ------------
 
 >**Note**
->Unless you're documenting a feature that's new to a specific version of Symfony
->(e.g. Symfony 2.3), all pull requests must be based off of the **2.2** branch,
->**not** the master or 2.3 branch.
+>Unless you're documenting a feature that was introduced *after* Symfony 2.3 
+>(e.g. in Symfony 2.4), all pull requests must be based off of the **2.3** branch,
+>**not** the master or older branches.
 
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 Symfony documentation, please read