[DataStore] Add README.md

Bug: 325144964
Fix: 325144964
Test: N/A
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bcb4a3a95d83a96ec7524dc27bb837db4f26ee70)
Merged-In: I1f88bf56361536b5e4fdec501f51de4639636556
Change-Id: I1f88bf56361536b5e4fdec501f51de4639636556
1 file changed