Update some READMEs under dev/. (#15712)

dev/README.md

+This directory contains tools and resources that the Flutter team uses
+during development of the framework. The tools in this directory
+should not be necessary for developing Flutter applications, though of
+course they may be interesting if you are curious.

dev/docs/README.md

-Flutter is Google’s mobile UI framework to craft high-quality native interfaces.
-Integrates with iOS and Android, works with existing code,
-and is free and open source.
-
-See the [getting started guide](https://flutter.io/getting-started/) for
-information about using Flutter.
+This directory contains the documentation generator for
+<https://docs.flutter.io/>. Those docs are updated only for the beta
+branch, see <../bots/docs.sh>. You can find docs for the master branch
+staged at <https://master-docs-flutter-io.firebaseapp.com/>.