Fix filepaths to be windows compatible

I had trouble using this with Windows (heh, I know) but it was simply the filepath was wrong when sending to Shopify yielding a 404.

instead of 

    assets/page.liquid 

windows would use 

    \assets\page.liquid

I put in a simple string replace to to fix and it is working great, 

Also, I just tweaked the readme to add a little clarity for the 'base' option as that tripped me up for a few minutes.