Updated migration guide

Documentation/Migration_3.0.md

@@ -1,6 +1,12 @@
-# Migrating from 2.0.0 to 3.0.0-beta.1
+# Migrating from 2.0.0 to 3.0.0-beta.3
 
 - The 'Box' dependency is no longer needed. If you're using a dependency manager, you'll probably won't have to do anything.
 - The 'Result' dependency has been updated to a newer version. As a result the static `Result.success` and `Result.failure` methods are no longer available. Use `Result(value:)` and `Result(error:)` instead.
+- The static methods on `Future` to create a Future are removed in favor of new initializers: `Future.succeeded` becomes `Future(value:)`, `Future.failed` becomes `Future(failure:)`, `Future.never` becomes `Future()`.
+- `success`, `failure` and `complete` now declare to throw an error. An error is thrown if any of those methods are called on an already completed future. Every method also has a `try-` variant (e.g. `trySuccess`) that never throws but instead returns a Bool indicating if the future was not yet completed.
+- It should no longer be needed to pass the execution context with an explicit parameter name. E.g. `onSuccess(context: q) {` now is `onSuccess(q) {`
+- Many free functions that used to take a Future (e.g. `flatten`, `promoteError`, `promoteValue`) are now methods on the Future itself (thanks to protocol extensions)
 
 There are some slight syntactical changes going from Swift 1.2 to Swift 2. Most of them can be fixed by Xcode automatically.
+
+Tip: use GitHub to generate a diff between the version you're using and the version you want to upgrade to and check out the changes made to the tests.