Update changelog with cache key caveat

Author: calvincestari

CHANGELOG.md

@@ -1,7 +1,7 @@
 # Change log
 
 ## v0.51.0
-- **Allow periods in arguments to be ignored when parsing cacheKeys**: If your query arguments include periods they will no longer cause broken cache keys. This means the cached data for those queries can be correctly found and returned. [#2057](https://github.com/apollographql/apollo-ios/pull/2057) - _Thanks to [Hesham Salman](https://github.com/Iron-Ham) for the contribution!_
+- **Allow periods in arguments to be ignored when parsing cacheKeys**: If your query arguments include periods they will no longer cause broken cache keys. This means the cached data for those queries can be correctly found and returned. The caveat with this change though is that if you use a persisted cache, after the upgrade you could see cache misses the data would need to be refetched. [#2057](https://github.com/apollographql/apollo-ios/pull/2057) - _Thanks to [Hesham Salman](https://github.com/Iron-Ham) for the contribution!_
 
 ## v0.50.0
 - **Dropped SPM support for Swift 5.2**: The minimum version of the Swift tools and language compatibilty required to process the SPM manifest is Swift 5.3. This means a minimum of Xcode version 12 is required for Swift Package Manager support. [#1992](https://github.com/apollographql/apollo-ios/pull/1992)