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/wasm-assemblyscript.md

@@ -0,0 +1,53 @@
+# AssemblyScript
+
+[AssemblyScript] 0.10.0 includes support for targeting WASI. To use it, add
+`import "wasi"` at the top of your entrypoint file.
+
+To create a program which can be run directly as a command, pass `--runtime half`
+to the AssemblyScript linker. This selects the [half runtime], which ensures that
+the generated wasm module doesn't contain any extraneous exports. (This isn't
+strictly required today, but the handling of extraneous exports may change in
+the future, so it's encouraged. As a bonus, it also reduces code size.)
+
+To create a program which can be loaded as a library and used from other modules,
+no special options are needed.
+
+Let's walk through a simple hello world example.
+
+## `wasi-hello-world.ts`
+
+```typescript
+{{#include ./assemblyscript-hello-world/wasi-hello-world.ts}}
+```
+
+This uses [as-wasi] as a dependency to make working with the AssemblyScript WASI
+bindings easier. Then, you can run:
+
+```sh
+asc wasi-hello-world.ts -b wasi-hello-world.wasm
+```
+
+to compile it to wasm, and
+
+```sh
+wasmtime wasi-hello-world.wasm
+```
+
+to run it from the command-line. Or you can instantiate it using the [Wasmtime API].
+
+## `package.json`
+
+It can also be packaged using a `package.json` file:
+
+```json
+{{#include ./assemblyscript-hello-world/package.json}}
+```
+
+You can also [browse this source code online][code] and clone the wasmtime
+repository to run the example locally.
+
+[code]: https://github.com/bytecodealliance/wasmtime/blob/master/docs/assemblyscript-hello-world
+[AssemblyScript]: https://assemblyscript.org
+[as-wasi]: https://github.com/jedisct1/as-wasi
+[half runtime]: https://docs.assemblyscript.org/details/runtime#runtime-variants
+[Wasmtime API]: ./lang.md