1151: Start writing the contribution guidelines r=mergify[bot] a=kaiyou
## What type of PR?
Documentation
## What does this PR do?
It provides a first suggested version of contribution guidelines for the project. These are very open do discussion. This was suggested in #1133.
## Prerequistes
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: kaiyou <pierre@jaury.eu>