Adds new query for subunit column and modifies ranks

* Adds new rank 9 for subunits in ne_adm0 dataset.
* Modifies manual synonyms to be rank 10.
* Updates documentation and procedures of the README.md file.

The manual synonyms table already has all the ranks set to 10 and the synonym table has been updated. 

I'd want to highlight that countries like South Korea or South Sudan, with which we were having problems, are appearing now.