Update gatsby dev readme

packages/gatsby-dev-cli/README.md

@@ -1,42 +1,52 @@
 # gatsby-dev-cli
 
-A commandline tool for local Gatsby development. When doing development work on the Gatsby core this tool allows you to easily update any project that has depencies on packages from Gatsby, like a site you are working on.
-
+A command-line tool for local Gatsby development. When doing development work
+on Gatsby core, this tool allows you to easily copy the changes to the various
+Gatsby packages to Gatsby sites that you're testing your changes on.
 
 ## Install
 
-```bash
-$ npm install -g gatsby-dev-cli@canary
-```
+`npm install -g gatsby-dev-cli@canary`
 
 or
 
-```bash
-$ yarn global add gatsby-dev-cli@canary
-```
+`yarn global add gatsby-dev-cli@canary`
 
 ## Configuration / First time setup
 
-The gatsby-dev-cli tool needs to know where your cloned Gatsby repository is located. You typically only need to configure this once.
+The gatsby-dev-cli tool needs to know where your cloned Gatsby repository is
+located. You typically only need to configure this once.
 
-```
-$ gatsby-dev --set-path-to-repo /path/to/my/cloned/version/gatsby
-```
+`gatsby-dev --set-path-to-repo /path/to/my/cloned/version/gatsby`
 
 ## How to use
 
-Navigate to the project you want to link to your forked Gatsby repository and run:
+Navigate to the project you want to link to your forked Gatsby repository and
+run:
+
+`gatsby-dev`
+
+The tool will then scan your project's package.json to find its Gatsby
+dependencies and copy the latest source from your cloned version of Gatsby into
+your project's node_modules folder. A watch task is then created to re-copy any
+modules that might change while you're working on the code, so you can leave
+this program running.
+
+Typically you'll also want to run `npm run watch` in the Gatsby repo to set up
+watchers to build Gatsby source code.
+
+### Other commands
 
-```bash
-$ gatsby-dev
-```
+#### `--packages`
 
-The tool will then scan your project's package.json to find any Gatsby dependencies and copy them into your project's node_modules folder. A watch task is then created to re-copy any modules that might change while you're working on the code, so you can leave this program running.
+You can prevent the automatic dependencies scan and instead specify a list of
+packages you want to link by using the `--packages` option:
 
-### Repo without gatby dependencies
+`gatsby-dev --packages gatsby gatsby-typegen-remark`
 
-You can prevent the automatic depencencies scan and instead specify a list of packages you want to link by using the `--packages` option:
+#### `--scan-once`
 
-```bash
-$ gatsby-dev --packages gatsby gatsby-typegen-remark
-```
+With this flag, the tool will do an initial scan and copy and then quit. This
+is useful for setting up automated testing/builds of Gatsby sites from the latest
+code. Gatsby's main website (and example websites) are built from HEAD using this
+flag, see https://github.com/gatsbyjs/gatsby/blob/1.0/scripts/publish-site.sh.