Update README.md

Author: nilportugues

README.md

@@ -31,7 +31,9 @@ Currently the following transformers make use of this library as foundation:
 
 To run the PHPUnit tests at the command line, go to the tests directory and issue phpunit.
 
-This library attempts to comply with PSR-1, PSR-2, and PSR-4. If you notice compliance oversights, please send a patch via pull request.
+This library attempts to comply with [PSR-1](http://www.php-fig.org/psr/psr-1/), [PSR-2](http://www.php-fig.org/psr/psr-2/), [PSR-4](http://www.php-fig.org/psr/psr-4/)and [PSR-7](http://www.php-fig.org/psr/psr-7/).
+
+If you notice compliance oversights, please send a patch via pull request.
 
 
 ## Author