Add a section to the users guide about deriving traits

[fitzgen]

Should include info on:

• How to enable non-default trait deriving

• Heads up about situations in which we can't add derive(Trait) because, eg there is a floating point member in a struct so Eq can't be derived

• Information about the --impl-blah options for when we can impl but can't derive

• How to disable trait deriving/impling for specific types