docs: create project folder in global installation (close #2045) (#2051)

packages/docs/docs/guide/getting-started.md

@@ -12,6 +12,9 @@ If you only want to play around with VuePress, you can install it globally:
 # install globally
 yarn global add vuepress # OR npm install -g vuepress
 
+# create the project folder
+mkdir vuepress-starter && cd vuepress-starter
+
 # create a markdown file
 echo '# Hello VuePress' > README.md