Devise Beluga onboarding process

[hidmic]

Feature description

Precisely what the title says. While we do have contributing guidelines, we don't have a good onboarding guide for anyone that may want to contribute to the project. What to read, what to try, where to start contributing, are all things that so far we have managed ad-hoc for a few Ekumen contributors. We need a guide, but most importantly we need a process, specially for the least experienced folks.