Added installation instructions

Signed-off-by: Thomas Welton <thomas@clicklabs.co.uk>
Updated manual install instructions

Based on comments from @aFarkas in pull request https://github.com/thomaswelton/html5shiv/commit/d4a6911dc4be86cd2c9d86b3d41d522ba5d9ca71#commitcomment-3162009
Merge pull request #101 from thomaswelton/readme

Added installation instructions