Adding download links to Zinc download page.
Updating Zinc download page to show version 3.0.0 release.
Added text on alternative download location for OpenCMISS-Zinc.
Added blurb from Richard on the Introduction to OpenCMISS-Zinc document with a link.
Merge pull request #50 from hsorby/master

Adding dowload links to Zinc 3.0.0 and blurb from Richard on Zinc introduction.