Fix README instructions (#204)

`themes` directory is already set in the submodule command. Therefore, you should not have it as working directory.

README.md

 
 Inside the folder of your Hugo site run:
 
-    $ cd themes
     $ git submodule add https://github.com/budparr/gohugo-theme-ananke.git themes/ananke
 
 For more information read the official [setup guide](//gohugo.io/overview/installing/) of Hugo.