Add contribution guidelines

[shtlrs]

Closes #75

This is just a draft as I don't know what exactly is supposed to be detailed into the contribution guideline, and also how detailed it needs to be without boring people ou.

@supakeen This one will need feedback and preferably more explicit details about how we'd want to this/what we want to include, and I'll update it accordingly.

[shtlrs]

@supakeen Do you think we should include a section where we detail how to choose an issue to work on ? I don't know how you usually prefer things to get done, meaning whether you prefer that people ask if they can work on an issue or not and have them assigned, or if people can just grab an unassigned issue & go for it.

[shtlrs]

@supakeen do you want me to keep this or shall I close it?

[supakeen]

@shtlrs Keep it, I'll take a look tomorrow (or today) (thanks for the ping).

[supakeen]

Actually this is good as-is; we can iterate on it.