Create CHANGELOG.md (#41)

.github/CHANGELOG.md

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+# Changelog
+## v0.0.1
+This release launches the Challenger Service.
+
+* [#19](https://github.com/bnb-chain/greenfield-challenger/pull/19) fix: added error logs  
+* [#20](https://github.com/bnb-chain/greenfield-challenger/pull/20) fix: fix hash verifier issue
+* [#21](https://github.com/bnb-chain/greenfield-challenger/pull/21) refactor: refine some codes
+* [#22](https://github.com/bnb-chain/greenfield-challenger/pull/22) refactor: refine codes to skip events and query heartbeat interval from blockchain
+* [#23](https://github.com/bnb-chain/greenfield-challenger/pull/23) add lint and gosec workflows
+* [#24](https://github.com/bnb-chain/greenfield-challenger/pull/24) doc: add deployment documentation 
+* [#25](https://github.com/bnb-chain/greenfield-challenger/pull/25) license: add opensource license 
+* [#28](https://github.com/bnb-chain/greenfield-challenger/pull/28) fix: fix lock issue 
+* [#29](https://github.com/bnb-chain/greenfield-challenger/pull/29) fix: fix some bugs 
+* [#30](https://github.com/bnb-chain/greenfield-challenger/pull/30) fix: change hash verification procedure 
+* [#31](https://github.com/bnb-chain/greenfield-challenger/pull/31) fix: add heartbeat interval to hash verifier 
+* [#32](https://github.com/bnb-chain/greenfield-challenger/pull/32) fix: return error if 2/3 votes have not been achieved 
+* [#34](https://github.com/bnb-chain/greenfield-challenger/pull/34) fix querying cached validators out of range error 
+* [#36](https://github.com/bnb-chain/greenfield-challenger/pull/36) feat: added vote collector, refactored broadcast 
+* [#38](https://github.com/bnb-chain/greenfield-challenger/pull/38) feat: refactor tx submitter 
+* [#39](https://github.com/bnb-chain/greenfield-challenger/pull/39) chore: update cosmos sdk to v0.47 
+* [#40](https://github.com/bnb-chain/greenfield-challenger/pull/40) fix: implement fix for issues found during qa testing