add note about adding empty directories to git

[robertfausk]

Git won't add empty directories but to take template override into account there is at least one of [Doctrine, DoctrineODM, Propel] needed. also see: https://github.com/symfony2admingenerator/AdmingeneratorGeneratorBundle/issues/789

[bobvandevijver]

I would use .empty or similair as filename, as .gitignore might confuse people.

[fadoe]

I prefer .gitkeep: http://stackoverflow.com/questions/7229885/what-are-the-differences-between-gitignore-and-gitkeep

[ioleo]

i also use .gitkeep

[robertfausk]

Changed .gitignore to .gitkeep

[ioleo]

merged, thanks