Add changelog format guide to CONTRIBUTING.md

Follow-up for #754.
Merge pull request #758 from yujinakayama/changelog-format-guide

Add changelog format guide to CONTRIBUTING.md
Support Rainbow gem both 1.99.x and 2.x

This fixes #762.
Fix typo with lowercase v for verbose output
Merge pull request #763 from yujinakayama/support-rainbow-2

Support Rainbow gem both 1.99.x and 2.x
Fix incorrect check for existence of an instance method
[Fix #714] New cop RequireParentheses.