Contributing guidelines

[CommonClimate]

In response to issue #110:
What should go into our contributing guidelines? What classes of users do we want to aim at? (novice, intermediate, advanced). Topics to hash out:

• design philosophy
• documenting new methods and classes (notebooks vs docstrings + Sphinx)
• naming conventions
• when to create a new class?
• continuous Integration and unit tests
• incorporating poorly supported packages (or not)
• when to create Pull Requests