Mention the ToS on our README (flutter#89765)

Author: Hixie

README.md

@@ -21,6 +21,12 @@ For announcements about new releases and breaking changes, follow the
 mailing list or see the
 [breaking changes](https://flutter.dev/docs/release/breaking-changes) page.
 
+## Terms of service
+
+The Flutter tool may occasionally download resources from Google servers. By downloading or using the Flutter SDK you agree to the Google Terms of Service: https://policies.google.com/terms
+
+For example, when installed from GitHub (as opposed to from a prepackaged archive), the Flutter tool will download the Dart SDK from Google servers immediately when first run, as it is used to execute the "flutter" tool itself. This will also occur when Flutter is upgraded (e.g. by running the "flutter upgrade" command).
+
 ## About Flutter
 
 We think Flutter will help you create beautiful, fast apps, with a productive,