Missing CONTRIBUTING file

```
| question | answer |
|---|---|
| backward compatibility? | yes |
| new feature? | no |
| bug fix? | no |
| new deprecations? | no |
| refactoring? | no |

PR Description

 - [ ] stuff to do
 - [ ] test
 - [ ] changelog
 - [ ] squash
 - [ ] merge
 - [ ] delete branch
```