Include getting started and contributing headings into Readme.

[shinjonathan]

[KianBadie]

I added a getting started section here by modifying the config/usage section to be subsections of the getting started section. I also added some details in order to make the new getting started section a bit more helpful in understanding how the mixin is setup to be used. Is this satisfactory for a getting started section?

In addition, I added a short section here that points to the contributing document.

If both of these additions are satisfactory, I will close this issue. If not, please let me know!