Add section to implementation book about providers

1) With 1.9 there is now an explicit Provider object in openmrs (previously it was just a User object). Add a section about how to add/edit providers and how they can be linked to a Person if need be

2) Along with the new provider objects the Encounter object now has a list of Providers (instead of having a single User provider). Mention this and how to store/retrieve/display that info