Add npm install --save-dev 🐖

Am I the only one who always forgets if it's --saveDev or --save-dev? My suggestions add --save-dev too. 💩

pages/common/npm.md

@@ -11,10 +11,14 @@
 
 `npm install`
 
-- Download a given dependency, and add it to the package.json:
+- Download a given dependency, and add it to the package.json (as `dependencies`):
 
 `npm install {{module_name}}@{{version}} --save`
 
+- Download a given dependency, and add it to the package.json (as `devDependencies`):
+
+`npm install {{module_name}}@{{version}} --save-dev`
+
 - Uninstall a module:
 
 `npm uninstall {{module_name}}`