Rewrite the README using Webpack

Right now Webpack probably has the most mature support for loading wasm modules,
so let's show off how to do that! Additionally this commits hello world as an
example to the repository.

examples/hello_world/index.js

@@ -0,0 +1,5 @@
+const js = import("./hello_world");
+
+js.then(js => {
+  js.greet("World!");
+});