message: add gatsby cli install docs
diff: @@ -43,7 +43,13 @@ A common question asked about Primer Components is how to know what should be ad
  We primarily use our documentation site as a workspace to develop new components or make changes to existing components (stay tuned for a better development environment coming soon!).
  -To get the documentation site running locally run the following in your terminal:
  +Before running the documentation site locally you'll need to install packages and make sure you've got the [Gatsby cli](https://www.gatsbyjs.com/docs/gatsby-cli/) installed:
  +
  +```sh
  +yarn && yarn global add gatsby-cli
  +```
  +
  +Then run the following to start up the site:
  ```sh
  yarn start
  