Removed outdated bs2/bs3 support info in readme

The readme on master claims that form-fields for bs2 and bs3 are supported, but in commit 
[Removed bs2 specific stuff completely]
the corresponding files were deleted, resulting in dead links. 
And we had a messed up wording:

>[...]you might want to set the 

>We have several files seperated[...]

[Removed bs2 specific stuff completely]:https://github.com/phiamo/MopaBootstrapBundle/commits/528cb460ea3d8e31152feb28941ddb94bf99567f/Resources/views/Form?page=2