Contibuting Guidelines

Found myself installing wrong (and totally unnecessary since it's already included with the project) version of Bourbon, so I decided to create contributing guidelines with several options to help future people get straight to the interesting part of contributing.

This PR is mostly an opportunity to start the conversation about the topic of contributions, so I'm sorry for impersonating Travis in `CONTIBUTING.md`, just wanted to have a general outline of how it might look like.

Vagrant and Bundler are more robust solutions to setting up development environment, but since at this stage Artists Theme relies only on Jekyll it would be nice and probably sufficient to just mention the version used.