Force to set default_language and language_override when creating index

[gabrielstuff]

Hello,

While creating text index for fields, I was not able to do it without specifying { default_language: 'en', language_override: 'en' }

In the documentation, you appear to simply tell the user to:

service.Model.createIndex({ username: 'text' }) which trows an error.

Maybe it is related to mongo version, but I suggest to explain a bit more this part.

Thanks for the great work