Updated root README.md

I have found the current root README.md file to be a little drab and uninformative, especially since it's the first point of contact with the OpenCV library when you go to the github page. So I've added a bit more information.

In particular, I've updated the file to:
- be more informative about the library's goals to new users
- contain quick links to installation guides
- improve the formatting slightly
- improve keywords for SEO
- be more in line with the goal of a README

I constructed the blurb with bits and pieces from <http://opencv.org/about.html>.