Add instructions for yarn install

Summary:
If you have not build the website before, you need to run `yarn install` first. Otherwise, the build script cannot find the docusaurus executable listed in `package.json` file.

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: IanChilds

Differential Revision: D57207975

fbshipit-source-id: c5ea62e0d5385fa541fd12c26a4c981ce19b893b

website/README.md

@@ -8,6 +8,7 @@ static website generator.
 ```shell
 $ yarn global add node-gyp
 $ yarn
+$ yarn install
 ```
 
 If on Eden you might get faster builds by doing