Testing instructions readme

I had hard time to work with components locally, when I followed the steps from https://github.com/mobify/stencil/issues/66 and I finally figured out. I thought it will be good idea to include to README. 
@kpeatt - I added PR for `grunt serve`, https://github.com/mobify/stencil/pull/107. I will need to update this README when 107 PR is merged.