Add README for gatsby-cli

packages/gatsby-cli/README.md

@@ -1,3 +1,34 @@
 # gatsby-cli
 
-Stub README
+Gatsby command line tool.
+
+Let's you create new Gatsby sites using [Gatsby starters](https://www.gatsbyjs.org/docs/gatsby-starters/)
+
+Also let's you run commands on sites. The tool runs code from the `gatsby` package
+installed locally.
+
+## Install
+
+`npm install --global gatsby-cli`
+
+## How to use
+
+Run `gatsby --help` for full help.
+
+### New
+
+`gatsby new gatsby-site`
+
+See the [Gatsby starters docs](https://www.gatsbyjs.org/docs/gatsby-starters/) for more
+
+### Develop
+
+At the root of a Gatsby site run `gatsby develop` to start the Gatsby development server.
+
+### Build
+
+At the root of a Gatsby site run `gatsby build` to do a production build of a site.
+
+### Serve
+
+At the root of a Gatsby site run `gatsby serve` to serve the production build of the site for testing.