Add 'How to Contribute' to the documentation

[oriordanemily]

We want Duplexity to be an open-source and contributable package. As such, I will do the following:

• Speak to David about best-practices for open source contribution
• Write a 'how to contribute' page in the docs for anyone that wishes to contribute

[oriordanemily]

Things that need adding to contribution guide:

• Documentation contribution
• Tests
• Style guides (e.g., using PEP8/black?)