Added migration guide for 1.15 (#3390)

* Added migration guide for 1.15

* wip

Author: mbrandonw

Sources/ComposableArchitecture/Documentation.docc/Articles/MigrationGuides.md

@@ -14,6 +14,7 @@ APIs, and these guides contain tips to do so.
 
 ## Topics
 
+- <doc:MigratingTo1.15>
 - <doc:MigratingTo1.14>
 - <doc:MigratingTo1.13>
 - <doc:MigratingTo1.12>

Sources/ComposableArchitecture/Documentation.docc/Articles/MigrationGuides/MigratingTo1.15.md

@@ -0,0 +1,49 @@
+# Migrating to 1.15
+
+The library has been completely updated for Swift 6 language mode, and now compiles in strict
+concurrency with no warnings or errors.
+
+## Overview
+
+The library is now 100% Swift 6 compatible, and has been done in a way that is full backwards
+compatible. If your project does not have strict concurrency warnings turned on, then updating
+the Composable Architecture to 1.15.0 should not cause any compilation errors. However, if you have
+strict concurrency turned on, then you may come across a few situations you need to update.
+
+### Enum cases as function references
+
+It is common to use the case of enum as a function, such as mapping on an ``Effect`` to bundle 
+its output into an action:
+
+```swift
+return client.fetch()
+  .map(Action.response)
+```
+
+In strict concurrency mode this may fail with a message like this:
+
+> 🛑 Converting non-sendable function value to '@Sendable (Value) -> Action' may introduce data races
+
+There are two ways to fix this. You can either open the closure explicitly instead of using 
+`Action.response` as a function:
+
+```swift
+return client.fetch()
+  .map { .response($0) }
+```
+
+There is also an upcoming Swift feature that will fix this. You can enable it in an SPM package
+by adding a `enableUpcomingFeature` to its Swift settings:
+
+```swift
+swiftSettings: [
+  .enableUpcomingFeature("InferSendableFromCaptures"),
+]),
+```
+
+And you can [enable this feature in Xcode](https://www.swift.org/blog/using-upcoming-feature-flags/)
+by navigating to your project's build settings in Xcode, and adding a new "Other Swift Flags" flag:
+
+```
+-enable-upcoming-feature InferSendableFromCaptures
+```