Merge add and add-dev and remove ls

pages/common/yarn.md

@@ -10,22 +10,14 @@
 
 `yarn`
 
-- Install a module and save it as a dependency to the package.json:
+- Install a module and save it as a dependency to the package.json. Add --dev to save as a dev dependency:
 
 `yarn add {{module_name}}@{{version}}`
 
-- Install a module and save it as a dev dependency to the package.json:
-
-`yarn add {{module_name}}@{{version}} --dev`
-
 - Uninstall a module and remove it from the package.json:
 
 `yarn remove {{module_name}}`
 
-- List a tree of installed modules:
-
-`yarn ls`
-
 - Interactively create a package.json file:
 
 `yarn init`