docs(contributing): Rewrite contributing
docs(contributing): Rewrite contributing

I added a template and a scheme section. I added a section about issues and PR. I added a lot of
others cool stuffs.

base16-builder/base16-builder,aloisdg/base16-builder,base16-builder/base16-builder,aloisdg/base16-builder,base16-builder/base16-builder,aloisdg/base16-builder