Drop most of the README (dart-archive#177)

We don't expect these rules to be usable today and we are not actively
working towards that.

Author: natebosch

README.md

@@ -1,47 +1,5 @@
 # Dart rules
 
-**WARNING** These are in active development and are *not* production-ready.
-Expect frequent breaking changes. The only way these rules should be used is
-with automatically generated BUILD files which should not be checked in since
-they will not be stable across versions of `rules_dart`.
-
-## Overview
-
-These build rules are used for building [Dart][] projects with Bazel. To use the
-Dart rules see [bazelify][].
-
-[Dart]: https://dartlang.org
-[bazelify]: https://github.com/dart-lang/bazel
-
-## Rules
-
-  * dart\_library
-  * dart\_vm\_binary
-  * dart\_vm\_snapshot
-  * dart\_vm\_test
-  * dart\_web\_application
-  * dart\_web\_test
-
-
-## Core rules
-
-`dart_library`: Declares a collection of Dart sources and data and their
-dependencies.
-
-
-## VM rules
-
-`dart_vm_binary`: Builds an executable bundle that runs a script or snapshot on
-the Dart VM.
-
-`dart_vm_snapshot`: Builds a VM snapshot of a Dart script. **WARNING** Snapshot
-files are *not* guaranteed to be compatible across VM releases.
-
-`dart_vm_test`: Builds a test that will be executed on the Dart VM.
-
-
-## Web rules
-
-`dart_web_application`: Compiles the specified script to JavaScript.
-
-`dart_web_test`: Builds a test that will be executed in the browser.
+**WARNING** These rules are not officially supported. Expect frequent breaking
+changes. Some of the rules have closed-source usages and may be updated without
+corrections in the open-source equivalents.