[Vertex AI] Add README for unit test mock files #9753

Job	Run time
Check changed files	0s
pod-lib-lint-abtesting (iOS)	7m 40s
pod-lib-lint-auth (iOS)	6m 43s
pod-lib-lint-database (iOS)	8m 3s
pod-lib-lint-dynamiclinks (iOS)	6m 34s
pod-lib-lint-firestore (iOS)	27m 39s
pod-lib-lint-functions (iOS)	2m 37s
pod-lib-lint-inappmessaging (iOS)	7m 47s
pod-lib-lint-messaging (iOS)	15m 37s
pod-lib-lint-performance (iOS)	9m 43s
pod-lib-lint-remoteconfig (iOS)	11m 28s
pod-lib-lint-storage (iOS)	7m 9s
create_report	1m 58s
	1h 52m 58s

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