Add documentation for building programs using AssemblyScript. (#1782)

* Add documentation for building programs using AssemblyScript.

* Add the assemblyscript hello world as an example and display it inline.

* Move the AssemblyScript hello world into the docs directory.

That way Cargo doesn't try to run it like a Rust example.

docs/assemblyscript-hello-world/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "wasi-hello-world",
+  "version": "1.0.0",
+  "description": "Hello world in Wasi with AS and as-wasi",
+  "main": "index.js",
+  "scripts": {
+    "build": "asc wasi-hello-world.ts -b wasi-hello-world.wasm -t wasi-hello-world.wat --runtime half",
+    "wasmtime": "wasmtime wasi-hello-world.wasm"
+  },
+  "author": "Aaron Turner",
+  "license": "MIT",
+  "devDependencies": {
+    "assemblyscript": "^0.10.0"
+  },
+  "dependencies": {
+    "as-wasi": "^0.1.1"
+  }
+}