Add documentation for bootstrap form theme

[lyrixx]

see symfony/symfony#10272 and symfony/symfony#12164

[simplyniceweb]

Hoping for a documentation soon. Thanks!

[henrikbjorn]

Why does it need documentation here? Theres already documentation for using a custom form layout.

[simplyniceweb]

Would you mind handing us the link?

Thanks!

[henrikbjorn]

http://silex.sensiolabs.org/doc/providers/twig.html there is a parameter called twig.form.templates

[lyrixx]

good point.

[fabpot]

I think that adding a note about the availability of the bootstrap theme with the path to use in the configuration would be great.

[simplyniceweb]

@fabpot i agree. after all newbie like me can make good use of it.

[henrikbjorn]

I dont see the need to copy paste some documentation that should be available in the symfony project. Otherwise we should really add a lot more notes everywhere about optional components and what they contain.

[fabpot]

see ca0677b