Adding migration guide for 1.6

docs/source/migrations/1.6.mdx

@@ -0,0 +1,52 @@
+---
+title: Apollo iOS 1.6 migration guide
+description: From 1.5 to 1.6
+---
+
+This guide describes the process of migrating your code from version 1.5 to version 1.6 of Apollo iOS. Please follow the relevant migration guides if you're on a version other than 1.5.
+
+## Apollo Codegen SPM Package
+
+The 1.6 update restructured the Apollo iOS ecosystem by splitting the code up into multiple different repositories/packages. One of these changes was the creation of the new `apollo-ios-codegen` repo and SPM package for users who want to do their code generation through Swift vs using the CLI tool.
+
+> **Note:** For more information on the project restructuring in the 1.6 release see [this](https://github.com/apollographql/apollo-ios/issues/3240) GitHub issue.
+
+If you are doing your code generation through Swift you have something like the following in your `Package.swift` file:
+
+```swift title="Package.swift"
+let package = Package(
+  name: "MyCodegen",
+  platforms: [.macOS(.v10_15)],
+  dependencies: [
+    .package(url: "https://github.com/apollographql/apollo-ios", exact: "1.5.0")
+  ],
+  targets: [
+    .executableTarget(
+      name: "MyCodegen",
+      dependencies: [
+        .product(name: "ApolloCodegenLib", package: "apollo-ios"),
+      ],
+      path: "Sources"),
+  ]
+)
+```
+
+In order to keep your code building successfully in the 1.6 release you will need to use the new `apollo-ios-codegen` package instead of the `apollo-ios` package:
+
+```swift title="Package.swift"
+let package = Package(
+  name: "MyCodegen",
+  platforms: [.macOS(.v10_15)],
+  dependencies: [
+    .package(url: "https://github.com/apollographql/apollo-ios-codegen", exact: "1.6.0")
+  ],
+  targets: [
+    .executableTarget(
+      name: "MyCodegen",
+      dependencies: [
+        .product(name: "ApolloCodegenLib", package: "apollo-ios-codegen"),
+      ],
+      path: "Sources"),
+  ]
+)
+```