Documentation: explain foreign keys support

[john8329]

Hi, long time user of go-pg/bun. It seems that one thing I've missed since the start are foreign keys. I thought the ORM doesn't support them, but digging in the code I found there's actually the WithForeignKeys() method.

I think it'd be super nice if a mention was made in the documentation. Being an ORM it relies heavily on conventions, and knowing what's the preferred way of handling these would help users being more productive.

Thanks