Upadate README.md for Symfony 4
Upadate README.md for Symfony 4

On Symfony 4, it seems that we should reference to a resource in a different way otherwise we get that error : 
Unable to find template "TetranzSelect2EntityBundle:Form:fields.html.twig"
tetranz/select2entity-bundle,tetranz/select2entity-bundle,tetranz/select2entity-bundle