minor #3546 Fix README: contributions should be based off 2.3 or higher (colinodell)

This PR was merged into the 2.3 branch.

Discussion
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Fix README: contributions should be based off 2.3 or higher

Per [the documentation](http://symfony.com/doc/current/contributing/documentation/overview.html#contributing) and [WouterJ's comment here](#3537 (comment)), the base branch should be 2.3, not 2.2.  This commit updates the README to reflect the language found in the documentation.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Commits
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ed5eab7 Contributions should be based off 2.3 or higher

Author: weaverryan

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.
 
 We love contributors! For more information on how you can contribute to the
 Symfony documentation, please read