Contribution guidelines enhancement

* Implements https://github.com/blog/1184-contributing-guidelines;
* Enhances the text in a way that allows even a user that is not familiarized with the tools to get started;
* Reinforces the need to provide a [Short, Self Contained, Correct Example](http://sscce.org/);
* Remove the text mentioning the github repo (sound redundand since this is already being read in the repository and npm shows the link to github)

@carhartl
Please, do not merge this before squashing, lets review.  
If you must, cherry pick the commited changes and commit into master, the github default merge operation is too verbose and add uneccessary commits to the master history.