Add explanation about why package:web_socket exists (#1207)

pkgs/web_socket/README.md

@@ -1,9 +1,31 @@
 [![pub package](https://img.shields.io/pub/v/web_socket.svg)](https://pub.dev/packages/web_socket)
 [![package publisher](https://img.shields.io/pub/publisher/web_socket.svg)](https://pub.dev/packages/web_socket/publisher)
 
-Any easy-to-use library for communicating with WebSockets that has multiple
+Any easy-to-use library for communicating with
+[WebSockets](https://en.wikipedia.org/wiki/WebSocket) that has multiple
 implementations.
 
+## Why another WebSocket package?
+
+The goal of `package:web_socket` is to provide a simple, well-defined 
+[WebSockets](https://en.wikipedia.org/wiki/WebSocket) interface that has
+consistent behavior across implementations.
+
+[`package:web_socket_channel`](https://pub.dev/documentation/web_socket_channel/)
+is currently the most popular WebSocket package. It has
+two implementations, one based on `package:web` and the other based on
+`dart:io` `WebSocket`. But those implementations do not have consistent
+behavior.
+
+[`WebSocket`](https://pub.dev/documentation/web_socket/latest/web_socket/WebSocket-class.html)
+currently has three implementations (with more on the way) that
+all pass the same set of
+[conformance tests](https://github.com/dart-lang/http/tree/master/pkgs/web_socket_conformance_tests):
+
+* [`BrowserWebSocket`](https://pub.dev/documentation/web_socket/latest/browser_web_socket/BrowserWebSocket-class.html)
+* [`CupertinoWebSocket`](https://pub.dev/documentation/cupertino_http/latest/cupertino_http/CupertinoWebSocket-class.html)
+* [`IOWebSocket`](https://pub.dev/documentation/web_socket/latest/io_web_socket/IOWebSocket-class.html)
+
 ## Using
 
 ```dart