Improve UPGRADING.md instructions for custom types
Improve UPGRADING.md instructions for custom types

* Provide an example for taking an existing type and porting it to
  the dry-types. This would have saved me some time.

* Removed examples of using `Virtus.model`. It seems confusing to
  support dry-types and Virtus at the same time.

Closes #2012

ruby-grape/grape,dblock/grape,intridea/grape