Changed the error message about the PHP requirement to go to STDERR
Merge pull request #445 from stof/patch-3

minor #445 Changed the error message about the PHP requirement to go to STDERR (stof)
Fixing readme generator

As mentioned in #397 [comment](https://github.com/fabpot/PHP-CS-Fixer/pull/397#commitcomment-7316182) I am trying to fix the readme generator. It seems to be working, but I guess it can be done better than replacing previously inserted `code-block:: bash`.

Now, there is adifferent problem. I deleted a blank lnie in FixCommand help message to avoid double inserts, but the fixer run on itself keeps adding a blank line before return even in a string.