Add a changelog entry for `Null`.

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04f687745d4a0e8638fc425d18e53b0206b2dda1..7f3f630a6de551e40497cefe7f990320914e8c40 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,20 @@
     #2      main (file:///Users/mit/tmp/tool/bin/main.dart:9:10)
     ```
 
+  * The `Null` type has been moved to the bottom of the type hierarchy. As such,
+    it is considered a subtype of every other type.

[Lasse Reichstein Nielsen, 2017/01/19 13:58:47]

(Not technically true, since bottom exists, but let's not confuse the reader: )

[floitsch, 2017/01/19 14:18:53]

Acknowledged.

+
+    Examples:
+    ```
+    Null foo() => null;
+    int x = foo();
+    String x = foo();
+
+    List<Null> bar() => [];

[Lasse Reichstein Nielsen, 2017/01/19 13:58:47]

Add the type parameter to the literal as well, just for good measure.

[floitsch, 2017/01/19 14:18:54]

Done.

+    List<int> = bar();
+    List<String> = bar();
+    ```
+
 ### Tool changes
 
 * Dart2Js