update contribution guide for 2.7/3.0
bug #4513 [Contributing] update contribution guide for 2.7/3.0 (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Contributing] update contribution guide for 2.7/3.0

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4508

Commits
-------

87582a4 update contribution guide for 2.7/3.0