Document high-level model building concepts

[ajcvickers]

Such as:

• Fluent over Attribute over Convention precedence for mapping
• Last-one-wins in Code First

[AndriySvyryd]

• If a metadata property/navigation name matches a CLR member it must be using it (can't be shadow).
• IModel vs IMutableModel vs IConventionModel vs ModelBuilder vs IConventionModelBuilder
• Show how to print out the Debug view of the model before and after model validation.
• ConventionBuilders throwing for invalid configuration and returning null for overriden configuration
• Metadata API have no side effects (apart from conventions), ConventionBuilders will try to make the adjustments to make the configuration valid, but will offload as much as possible to the conventions