This repository should provide a link to DML 1.4 reference manual and style guide

I've noticed its not really easy to access this manual by searching the web. I get some links to 1.2 reference manual, but given DMLC here does support 1.4 and the source is public, the language reference manual for 1.4 and the style guide should be accessible as well.

I suggest to find some way to distribute the manual and link it here in the repo, maybe in the README