Response constructor with ReadableStream.

This is an initial proposal for #63.

response-stream-constructor.md

@@ -0,0 +1,22 @@
+__ASSUME [body-readable-stream.md](body-readable-stream.md) is applied to [the fetch spec](https://fetch.spec.whatwg.org/).__
+
+## [Body mixin](https://fetch.spec.whatwg.org/#response-class) ##
+
+To _extract_ a body and a \`Content-Type\` value from _object_, run these steps:
+
+1. (Same)
+1. (Same)
+1. Switch on _object_'s type:
+ - ReadableStream => Set _stream_ to _object_.
+ - (The original algorithm follows...)
+
+## [Response class](https://fetch.spec.whatwg.org/#response-class) ##
+
+```
+typedef (Blob or BufferSource or FormData or URLSearchParams or USVString or ReadableStream) ResponseBodyInit;
+
+[Construct(ResponseBodyInit, option ResponseIniit init), Exposed=(Window,Worker)]
+interface Response {
+ ...
+}
+```