message: Updated CHANGELOG.md for v3.4.0
diff: ## Change Log
  +### v3.4.0 (2020-06-15)
  +- [#142](https://github.com/sapientglobalmarkets/react-querybuilder/pull/142) Implemented customizable Rule component (@jakeboone02)
  +
  ### v3.3.0 (2020-06-12)
  - [#141](https://github.com/sapientglobalmarkets/react-querybuilder/pull/141) Add custom control option for RuleGroup component (@rbalaine)
  