Update the readme for gatsby-dev-cli (#820)

Author: 0x80

packages/gatsby-dev-cli/README.md

@@ -1,35 +1,42 @@
 # gatsby-dev-cli
 
-A dev cli tool for Gatsby.
+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.
 
-It's a simple cli tool for have easily a functionnal development environment for working on Gatsby.
 
 ## Install
 
-Fork gatsby repo and clone it.
-
 ```bash
 $ npm install -g gatsby-dev-cli@canary
-$ gatsby-dev --set-path-to-repo /path/to/my/cloned/version/gatsby
 ```
 
+or
 
-## How to use
+```bash
+$ yarn global add gatsby-dev-cli@canary
+```
+
+## Configuration / First time setup
 
-Go to your project and execute corresponding following command to use local version of gatsby.
+The gatsby-dev-cli tool needs to know where your cloned Gatsby repository is located. You typically only need to configure this once.
 
-Note: First time can be very long, be patient ;)
+```
+$ gatsby-dev --set-path-to-repo /path/to/my/cloned/version/gatsby
+```
 
-### Repo with gatsby dependencies
+## How to use
+
+Navigate to the project you want to link to your forked Gatsby repository and run:
 
 ```bash
 $ gatsby-dev
 ```
 
-Tools automaticly find gatsby packages into current `package.json`, copy it from your local fork and watch!
+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.
 
 ### Repo without gatby dependencies
 
+You can prevent the automatic depencencies scan and instead specify a list of packages you want to link by using the `--packages` option:
+
 ```bash
 $ gatsby-dev --packages gatsby gatsby-typegen-remark
 ```