doc: Explain rebase/squash policy in CONTRIBUTING.md
doc: Explain rebase/squash policy in CONTRIBUTING.md

Former-commit-id: c7ad8d5a8b022c318a64d457c2e1edf48ac78115
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin