Update changelog for version 6.1 (#345)

Author: aarongable

CP-CPS.md

@@ -48,6 +48,7 @@ The following revisions have been made:
 | Jul 30, 2025 | Updates regarding MPIC, Short-lived Certificate revocation, Root CA Certificate profile, WebTrust audits, and HSM validation standards. Minor language consistency and clarity updates throughout. | 5.8 |
 | Aug 29, 2025 | Add mass revocation readiness language. | 5.9 |
 | Nov 5, 2025 | Add ISRG Root YE and Root YR. | 6.0 |
+| May 12, 2026 | Add Cross-Certified Subordinate CA Profile, minor clean-ups. | 6.1 |
 
 ## 1.3 PKI participants