Update README.md

Author: ohJohanZ

README.md

@@ -58,6 +58,7 @@ Some open security audit reports of SlowMist.
 * [Litentry-node Module Security Audit Report](./open-report/Litentry-node%20Module%20Security%20Audit%20Report.pdf)
 * [ZKSwap Security Audit Report](./open-report/ZKSwap-Security-Audit-Report.pdf)
 * [PlatON安全审计报告](./open-report/PlatON安全审计报告.pdf)
+* [Vexanium Chain Security Audit Report](./open-report/Vexanium-chain-Security-Audit-Report.pdf)
 
 ## 思维导图