Add RBS example

index.bs

@@ -287,12 +287,38 @@ copies.
   on streams, beyond the provided ones of <a>piping</a> and <a lt="tee a readable stream">teeing</a>.
 </div>
 
-<h3 id="rs-with-byte-source-intro">Using Readable Streams with Byte Source</h3>
+<div class="example">
+  The above example showed using the readable stream's <a>default reader</a>. If the stream is a <a>readable byte
+  stream</a>, you can also acquire a <a>BYOB reader</a> for it, which allows more precise control over buffer
+  allocation in order to avoid copies. For example, this code reads the first 1024 bytes from the stream into a single
+  memory buffer:
 
-<div class="note">
-  A readable stream may be able to provide a <code>ReadableStreamBYOBReader</code>. It has almost the same methods as
-  the <code>ReadableStreamDefaultReader</code>, and can be
-  read in the same manner as the <code>ReadableStreamDefaultReader</code>.
+  <pre><code class="lang-javascript">
+    const reader = readableStream.getBYOBReader();
+
+    let startingAB = new ArrayBuffer(1024);
+    readInto(startingAB, 0)
+      .then(buffer => console.log("The first 1024 bytes:", buffer))
+      .catch(e => console.error("Something went wrong!", e));
+
+    function readInto(buffer, offset) {
+      if (offset === buffer.byteLength) {
+        return buffer;
+      }
+
+      const view = new Uint8Array(buffer, offset, buffer.byteLength - offset);
+      reader.read(view).then(newView => {
+        return readInto(newView.buffer, offset + newView.byteLength);
+      });
+    }
+  </code></pre>
+
+  An important thing to note here is that the final <code>buffer</code> value is different from the
+  <code>originalAB</code>, but it (and all intermediate buffers) share the same backing memory allocation. At each step,
+  the buffer is transferred <!-- TODO link --> to a new <code>ArrayBuffer</code> object. The <code>newView</code> is a
+  new <code>Uint8Array</code>, with that <code>ArrayBuffer</code> object as its <code>buffer</code> property, the offset
+  that bytes were written to as its <code>byteOffset</code> property, and the number of bytes that were written as its
+  <code>byteLength</code> property.
 </div>
 
 <h3 id="rs-class">Class <code>ReadableStream</code></h3>