Placeholders, folder structure, readme

I made contributions to the following:

_readme_
Added basic overview of the repo with code example.

_directory structure_
I reorganised the folder structure. I thought that having two "grid" files might be confusing so I joined the two and deleted main.scss for clarity. I would suggest removing the src folder but my opinion is that it is not critical. 

_placeholders_
I added placeholders to reduce unnecessary overhead in post-compile code.