docs; make a note re: #2955 in migration guide

docs/migration.jade

@@ -27,6 +27,10 @@ block content
   :markdown
     In Mongoose 3, CastError and ValidationError had a `type` field. For instance, user defined validation errors would have a `type` property that contained the string 'user defined'. In Mongoose 4, this property has been renamed to `kind` due to [the V8 JavaScript engine using the Error.type property internally](https://code.google.com/p/v8/issues/detail?id=2397).
 
+  h3#promises Query now has a `.then()` function
+  :markdown
+    In mongoose 3, you needed to call `.exec()` on a query chain to get a promise back, like `MyModel.find().exec().then();`. Mongoose 4 queries are promises, so you can do `MyModel.find().then()` instead. Be careful if you're using functions like [q's `Q.ninvoke()`](https://github.com/kriskowal/q#adapting-node) or otherwise returning a mongoose query from a promise.
+
   h3#moreinfo More Info
   :markdown
     Related blog posts: