Document subtypes and deprecate old text ops

We've supported subtypes since https://github.com/ottypes/docs/pull/9.  This PR updates the readme with documentation for them, and deprecates the old text ops, which can now be represented as subtype ops.  IMO we should eventually completely remove the old string ops since they are internally converted to and from subtype ops anyway, but that would break backward compatibility.

btw, I'd have made a branch on the main project instead of a fork, but the "Collaborators" team on Github doesn't have write access to this (or any) repository. :smile: