Multilingual support for the documentation

[James Turton]

Hi all

Based on an initiative of Cong Luo's, I've just implemented support for 
doc pages in multiple languages on the Drill website.  There are 
currently a handful of farcically translated pages in Simplified Chinese 
which I put there for demonstration purposes and which can be found by 
using the new Language selector in the top menu.

Instructions for adding translated pages have been added to README.md in 
the gh-pages branch.  Now all that's missing is an army of translators 
so please encourage your multilingual friends.  With any luck this will 
remove a barrier to entry for many prospective Drill users.

Regards
James