Improve documentation for new contributors

[emilsoman]

This issue is on me. I need to talk to Fika contributors to understand the challenges they faced when getting started and improve the documentation/process around submitting a PR. All (potential) contributors are welcome to add comments on this issue if they have ideas on making it easier to get started with contributing to Fika.

• Add a PR template
• Add documentation on how to add tests
• Let contributors know who to get in touch with and how, regarding different parts of the code
• Add a wiki page about the vision, goals and design decisions of Fika