Add Design system notice message

From the end of June 2018, GDS will stop maintaining the code and guidance in GOV.UK Frontend Toolkit.

You will be able to find tools and helpers for
building frontend apps in the GOV.UK Design System.

GOV.UK Frontend Toolkit will still be available in case you are already using it.

README.md

@@ -1,3 +1,12 @@
+The GOV.UK Design System is launching 22 June
+===============
+
+When it's launched the Design System will contain the tools and helpers for building frontend apps.
+
+If you’re starting a new project and you’re able to wait until 22 June, we recommend using the Design System when it launches.
+
+GOV.UK Frontend Toolkit will still be available in case you are already using it.
+
 # GOV.UK frontend toolkit
 
 A collection of Sass and JavaScript files for using as part of your