Create a contribution guide

[luixxiul]

Is your feature request related to a problem? Please describe.

In order to welcome contributions from a lot of people, I would like to suggest to create a contribution guide. It might have been not necessary when this project has started as the Matrix ecosystem was constantly growing along with it, but since then several projects have stopped their development and some of them have been deprecated in favor of either its successors or other projects. IMHO it is important to keep the playbook up-to-date with the Matrix ecosystem, and for that object the more eyes and hands we have, the better it should be :)

Describe the solution you'd like

The contribution guide should have a section for:

• How to create a playbook for a new integration
• How to update the playbook if it gets out of date
• How to test and spot a bug
• How to improve documentation (including finding a typo)
• How to help other people by answering questions
• How to help other people who do not speak English

etc.

Create a CONTRIBUTING.md: #1657 (comment)

Describe alternatives you've considered

Additional context