Update changelog for GN change

R=asiva@google.com

Review-Url: https://codereview.chromium.org/2638383005 .

CHANGELOG.md

@@ -71,6 +71,17 @@
   * Make `pub run` run executables in spawned isolates. This lets them handle
     signals and use standard IO reliably.
 
+### Infrastructure changes
+
+  * The SDK now uses GN rather than gyp to generate its build files, which will
+    now be exclusively ninja flavored. Documentation can be found on our
+    [wiki](https://github.com/dart-lang/sdk/wiki/Building-with-GN). Also see the
+    help message of `tools/gn.py`. This change is in response to the deprecation
+    of gyp. Build file generation with gyp will continue to be available in this
+    release by setting the environment variable `DART_USE_GYP` before running
+    `gclient sync` or `gclient runhooks`, but this will be removed in a future
+    release.
+
 ## 1.21.0 - 2016-12-07
 
 ### Language