doc: Adds migration guide

MIGRATION.md

@@ -0,0 +1,39 @@
+# Migration
+
+## 1.0 to 2.0
+
+### TypeReference removal
+
+The `TypeReference` type was removed in v2.0.0, since it was made unnecessary when using the [GraphQL](https://github.com/GraphQLSwift/GraphQL)  closure-based `field` API. Simply replace any `TypeReference` usage with the actual type:
+
+```swift
+// Before
+let schema = try! Schema<HelloResolver, HelloContext> {
+    Type(Object1.self) { }
+    Type(Object2.self) {
+        Field("object1", at: \.object1, as: TypeReference<Object1>.self)
+    }
+}
+
+// After
+let schema = try! Schema<HelloResolver, HelloContext> {
+    Type(Object1.self) { }
+    Type(Object2.self) {
+        Field("object1", at: \.object1)
+    }
+}
+```
+
+### Field/InputField `as` argument removal
+
+Since TypeReference was removed, there is no longer a functional need for the `as` argument on `Field` and `InputField`
+types. To remove it, simply omit it and let the compiler determine the type from the signature of the `at` argument.
+
+### `Types` removal
+
+The deprecated `Types` type has been removed. Instead define types individually using the `Type` initializers.
+
+### Reflection `isEncodable`
+
+The deprecated `Reflection.isEncodable(type: Any.Type) -> Bool` function has been removed. Please re-implement in your
+package if this functionality is needed.