Add in Contributing.md that only compilation fixes are allowed in develop branch

Description

If develop branch is broken, it is simpler to just push the "COMP" fixes on develop directly.

We can agree that only COMP commits that fix the dashboard can go directly to develop. All the others go to merge request.

Steps to reproduce

Describe as precisely as possible how to reproduce the bug. Try to isolate a minimal number of steps. Also describe reproducibility (always, random ...).

Configuration information

OS, OTB version or tag, information related to build (binaries, superbuild, system libs ...)