update migration guide

closes actix#1680

MIGRATION.md

@@ -3,12 +3,23 @@
 
 ## 3.0.0
 
+* The return type for `ServiceRequest::app_data::<T>()` was changed from returning a `Data<T>` to
+  simply a `T`. To access a `Data<T>` use `ServiceRequest::app_data::<Data<T>>()`.
+
+* Cookie handling has been offloaded to the `cookie` crate:
+  * `USERINFO_ENCODE_SET` is no longer exposed. Percent-encoding is still supported; check docs.
+  * Some types now require lifetime parameters.
+
+* The time crate was updated to `v0.2`, a major breaking change to the time crate, which affects
+  any `actix-web` method previously expecting a time v0.1 input.
+
 * Setting a cookie's SameSite property, explicitly, to `SameSite::None` will now
   result in `SameSite=None` being sent with the response Set-Cookie header.
   To create a cookie without a SameSite attribute, remove any calls setting same_site.
 
 * actix-http support for Actors messages was moved to actix-http crate and is enabled 
   with feature `actors`
+
 * content_length function is removed from actix-http.
   You can set Content-Length by normally setting the response body or calling no_chunking function.