[DOC] Clarify CLA signing

[tbouffard]

The Contributing Guide explains why we require to Sign the Contributor License Agreement but the link to the document points to a 404.

Tasks to do:

• confirm that CLA signing is required: yes, this is required by @bonitasoft
• setup a solution to sign it. Done, via the cla-assistant app
• fix the link in the document and explain why this is required

CLA Automation solutions

• https://github.com/cla-assistant/cla-assistant: we plan to use this one
• https://github.com/cla-assistant/github-action

[tbouffard]

After discussions with @nchabanoles we agree to update the contributing guidewith the following text to explain why we request CLA signing:

By signing the CLA we acknowledge that your contribution is accepted as it and
that you cannot be held responsible for any impacts on its integration. You then
grant us the right to modify and distribute your code without restrictions. We
ask this of all contributors in order to assure our users of the origin and
continuing existence of the code.