Updated README.

 - Windows instructions: PPM repo is no longer up to date.
   Also, ActiveState now has SVG in its own repository.
 - Removed fifth way of 'installing' SVG'.
 - Removed URLs for related resources that were no longer working.
 - Removed reference to SVG::Manual, which was moved into SVG Pod.
Merge pull request #3 from mbeijen/master

Updated instructions in README