Update getting-started.md (#17)

I find  when I use index.md,  the serve will return a status of 404, but when I change to readme.md, it works

Author: wangyi7099

Diff for: docs/guide/getting-started.md

@@ -9,7 +9,7 @@ If you just want to play around with VuePress, you can install it globally:
 npm install -g vuepress
 
 # create a markdown file
-echo "# Hello VuePress!" > index.md
+echo "# Hello VuePress!" > README.md
 
 # start writing
 vuepress dev .