Update README to include additional installation instructions

Summary:
When I was setting up the github workflow, I remembered that there's a couple additional installation steps that's needed for someone with a fresh machine
- installing wasm-pack
- using rust nightly

Adding these steps to the README

Reviewed By: rchen152

Differential Revision: D70748541

fbshipit-source-id: 85500aa2f909c038373e784e93138de4be2fb33d

Author: dluo

website/README.md

@@ -6,13 +6,17 @@ static website generator.
 ## Local Development
 
 IMPORTANT: If you want to test any sandbox functionalities, you should be
-developing on your local machine. This is because we rely on cargo, which can
-only be built locally right now.
+developing on your local machine. This is because the sandbox relies on cargo,
+which can only be built locally right now. For Meta engineers who are not
+working on the sandbox, you can build on your dev machine using the following
+instructions.
 
 Install dependencies:
 
 ```bash
-$ yarn install
+$ yarn install # yarn dependencies
+$ cargo install wasm-pack # wasm-pack for wasm build
+$ rustup install nightly && rustup default nightly # the wasm build must be done on the rust nightly build
 ```
 
 Start a local development server: