Added license file

Please consider specifying the license under which corenlp_xml can be used, e.g. with a permissive license like the apache license that xtas is released under or a more "copyleft" license such as GPLv2. 

Including a line such as "Copyright Robert Elwell, distributed under the Apache License (http://www.apache.org/licenses/LICENSE-2.0)" in your readme.rst is presumably sufficient. See also: https://help.github.com/articles/open-source-licensing

I've taken the liberty to create a pull request for the apache license (see also: http://en.wikipedia.org/wiki/Apache_License)