Roll versions and add a new release_notes directory.

sky/sdk/pubspec.yaml

Index: sky/sdk/pubspec.yaml
diff --git a/sky/sdk/pubspec.yaml b/sky/sdk/pubspec.yaml
index b0ff7556cacfe29864e618b7b19430dd2792f547..e0e81898bef04afd0bad04e8b3531f848070ed44 100644
--- a/sky/sdk/pubspec.yaml
+++ b/sky/sdk/pubspec.yaml
@@ -1,14 +1,14 @@
 author: Chromium Authors <sky-dev@googlegroups.com>
 dependencies:
-  cassowary: '^0.1.7'
-  mojo_services: '^0.0.15'
-  mojo: '^0.0.17'
-  mojom: '^0.0.17'
-  newton: '^0.1.0'
-  vector_math: '^1.4.3'
+  cassowary: ^0.1.7
+  mojo: ^0.0.17
+  mojo_services: ^0.0.15
+  mojom: ^0.0.17
+  newton: ^0.1.0
+  vector_math: ^1.4.3
 description: Dart files to support executing inside Sky.
-homepage: https://github.com/domokit/mojo/tree/master/sky
 environment:
-  sdk: ">=1.8.0 <2.0.0"
+  sdk: '>=1.8.0 <2.0.0'
+homepage: https://github.com/domokit/mojo/tree/master/sky
 name: sky
-version: 0.0.17
+version: 0.0.18