update to readme

Seem like you need to install the dependencies before you can run `netlify plugins:link .`

Author: Phil Hawksworth

README.md

@@ -23,7 +23,8 @@ Then clone and activate the plugin:
 ```
 git clone [email protected]:netlify/netlify-dev-plugin.git
 cd netlify-dev-plugin
-netlify plugins:link .
+npm install
+c
 ```
 
 Now you're both ready to start testing netlify dev and to contribute to the project.