Updating the README file.

Summary: Just some updates to the content of the README file, since it's a bit out of date.  Included information on how to build and clarified that this isn't going to be a freeware game..

Test Plan: N/A; if you like the new README, accept the diff.

Reviewers: Sabre070

Reviewed By: Sabre070

Differential Revision: http://code.redpointsoftware.com.au/D21