Document RFC requirement

[RomainMuller]

Update the CONTRIBUTING.md file to reflect that any breaking change in jsii protocols (Assembly schema or semantics, runtimes behavior, ...) must go through the RFC process and be provided sufficient time & attention to assert that:

• Sufficient consideration was given to all alternatives to breaking, and that rejection of those has been sufficiently motivated
• Proactively documented a migration guide to help users affected by the breakage to move past it with as little effort as possible
• Have comprehensively documented the reasons why the breaking change happens and the though process that led to it

[github-actions]

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