Write and add a contribution guide to the readme

[iambrosi]

We need a few specs (like code style) to be added to a contribution guide, which should be taken into account at the moment of submitting a PRs.

[mtrias]

maybe also adding a .editorconf would help?

[iambrosi]

@mtrias :+1:

[TheSharpieOne]

How about .eslintrc (or your preferred linter rc file) ?

[mtrias]

Using eslint sounds good to me.