Use git submodules (#183)

As per https://gohugo.io/getting-started/quick-start/ I think it is better to use git submodules rather than cloning the repo. It should be easier to keep up to date etc. So please consider this change in documentation

README.md

 Inside the folder of your Hugo site run:
 
     $ cd themes
-    $ git clone https://github.com/budparr/gohugo-theme-ananke.git
+    $ 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.