Committing CHANGELOG.md for release 1.8.0 PC release (#1336)

Summary:
Pull Request resolved: #1336

For Bundle B638 which is in prod stage: https://www.internalfb.com/svc/services/private_measurement/private_computation_service/conveyor

Following wiki: https://www.internalfb.com/intern/wiki/Private_Computation_Infra/Private_Services_Infra/Release/Private_Lift_&_Attribution_release/

Reviewed By: gorel

Differential Revision: D38089605

fbshipit-source-id: 6d8127ec9c2ed834ff07cd9c7014e9b7d59da56d

fbpcs/CHANGELOG.md

@@ -15,6 +15,8 @@ Types of changes
 
 ## [Unreleased]
 
+## [1.8.0] - 2022-07-22
+
 ### Added
 - Added optional new argument --padding_size which is passed as multi_conversion_limit to the lift id spine combiner and as num_conversions_per_user to the pcf2 private lift stage