Mention CCLA and ICLA in Contribute.md

Description

Add information about CCLA and ICLA on

Add a tick button in the merge request template to check that the contributor has signed the CLA

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 ...)