Adding Github readme

Hi,

I've taken the liberty of adding a readme for the project to help people like me who prefer github to vimscripts.org.  This is a handy plugin so I think it should have all the publicity it can get.  You can see the rendered result at https://github.com/dwaller/vim-blockle/tree/jgdavey-repo

I've also correcting a couple of trivial typos in the help text while I was there.

David