added breaking change note to 3.8 release (parse-community#6023)

Breaking change as described here: parse-community#5983 (comment)

Author: mtrezza

CHANGELOG.md

@@ -38,6 +38,9 @@
 - FIX: Transaction was aborting before all promises have either resolved or rejected [#5878](https://github.com/parse-community/parse-server/pull/5878). Thanks to [Antonio Davi Macedo Coelho de Castro](https://github.com/davimacedo)
 - NEW: Use transactions for batch operation [#5849](https://github.com/parse-community/parse-server/pull/5849). Thanks to [Antonio Davi Macedo Coelho de Castro](https://github.com/davimacedo)
 
+#### Breaking Changes:
+- If you are running Parse Server on top of a MongoDB deployment which does not fit the [Retryable Writes Requirements](https://docs.mongodb.com/manual/core/retryable-writes/#prerequisites), you will have to add `retryWrites=false` to your connection string in order to upgrade to Parse Server 3.8.
+
 ### 3.7.2
 [Full Changelog](https://github.com/parse-community/parse-server/compare/3.7.1...3.7.2)