Additional Web API 2 to MVC 6 Migration content

[danroth27]

• Migrating HttpConfiguration logic to the MvcOptions pattern
• Migrating message handlers to middleware
• Migrating formatters to the new formatter model
• Using the Microsoft.AspNet.Mvc.WebApiCompatShim package if you are porting a lot of Web API 2 code and you want to use the Web API 2 patterns (method overloading, HTTP verb convention based on method name, HttpRequestMessage/HttpResponseMessage programming model, etc.). Web API compatibility shim WebAPI #1811

[tdykstra]

@danroth27 If the new custom formatters doc addresses "migrating formatters", we can close this old issue.

[Rick-Anderson]

#1811 tracked.