Add a quick migrations guide to the README
Add a quick migrations guide to the README

This should be expanded further, but I want to scrap most of the README
and do a better guide (e.g. I don't think we need to mention `table!`,
at least not before mentioning `infer_schema!()`)

/cc @mfpiccolo

sgrif/diesel,mcasper/diesel,sgrif/diesel,diesel-rs/diesel,cascalheira/diesel,tohou/diesel,mcasper/diesel,diesel-rs/diesel,SoftwareWarlock/diesel,cascalheira/diesel,SoftwareWarlock/diesel,tohou/diesel