Add section on dropping DelayedInit

Author: odersky

docs/docs/reference/delaydInit.md

@@ -0,0 +1,26 @@
+---
+layout: doc-page
+title: Dropped: Delayedinit
+---
+
+The special handling of the `DelayedInit` trait is no longer
+supported.
+
+One consequence is that the `App` class, which used `DelayedInit` is
+now partially broken. You can still use `App` for an easy and concise
+way to set up a main program. Example:
+
+    object HelloWorld extends App {
+      println("Hello, world!")
+    }
+
+However, the code is now run in the initializer of the object, which on
+some JVM's means that it will only be interpreted. So, better not use it
+for benchmarking! Also, if you want to access the command line arguments,
+you need to use an explicit `main` method for that.
+
+    object Hello extends App {
+      def main(args: Array[String]) =
+        println(s"Hello, ${args(0)}")
+    }
+

docs/sidebar.yml

@@ -3,6 +3,10 @@ sidebar:
       url: blog/index.html
     - title: Reference
       subsection:
+        - title: Dropped Features
+          subsection:
+            - title: DelayedInit
+            - url: docs/reference/delayed-init.md
         - title: Implicit Function Types
           url: docs/reference/implicit-function-types.md
         - title: Intersection types