Add a rough guide for updating
Add a rough guide for updating

As I wrote that example, it occurs to me that having a single struct
that implements both `Queriable` and `AsChangeset`, where we update and
then mutate with the changes as returned by the DB is a useful pattern
that I should support in the library itself.

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