src/examples/wasm: Show both methods supported

Adds another example showing the simple case
of executing main, adds a README explaining how
everything fits together and how to execute the compiled
code in the browser. Include a minimal webserver for
local testing.

src/examples/wasm/export/wasm.js

@@ -0,0 +1,35 @@
+'use strict';
+
+const WASM_URL = 'wasm.wasm';
+
+var wasm;
+
+function updateResult() {
+  wasm.exports.update();
+}
+
+function init() {
+  document.querySelector('#a').oninput = updateResult;
+  document.querySelector('#b').oninput = updateResult;
+
+  const go = new Go();
+  if ('instantiateStreaming' in WebAssembly) {
+    WebAssembly.instantiateStreaming(fetch(WASM_URL), go.importObject).then(function (obj) {
+      wasm = obj.instance;
+      go.run(wasm);
+      updateResult();
+    })
+  } else {
+    fetch(WASM_URL).then(resp =>
+      resp.arrayBuffer()
+    ).then(bytes =>
+      WebAssembly.instantiate(bytes, go.importObject).then(function (obj) {
+        wasm = obj.instance;
+        go.run(wasm);
+        updateResult();
+      })
+    )
+  }
+}
+
+init();