Release note for facebook#11738 AutoHCC (facebook#11855)

Summary:
Neglected in original PR

Pull Request resolved: facebook#11855

Test Plan: no code change

Reviewed By: jowlyzhang

Differential Revision: D49424759

Pulled By: pdillinger

fbshipit-source-id: 9a2ef2c0992a65f2a8f04dce9cefe978f8087407

unreleased_history/new_features/auto_hcc.md

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+Added an experimental new "automatic" variant of HyperClockCache that does not require a prior estimate of the average size of cache entries. This variant is activated when HyperClockCacheOptions::estimated\_entry\_charge = 0 and has essentially the same concurrency benefits as the existing HyperClockCache.