Add instructions on how to stop the Jekyll server

Author: whatyouhide

README.md

@@ -66,7 +66,8 @@ In order to run a development server (with live-reloading on) just run:
 $ bundle exec jekyll serve
 ```
 
-The generated site will be available at http://localhost:4000.
+The generated site will be available at http://localhost:4000. You can stop the
+server with <kbd>Ctrl</kbd>-<kbd>C</kbd>.
 
 ##### 5. Make your changes and push them