doc(indexeddb): add changelog file

Signed-off-by: Michael Goldenberg <m@mgoldenberg.net>

Author: mgoldenberg

crates/matrix-sdk-indexeddb/changelog.d/6872.fixed.md

@@ -0,0 +1,5 @@
+Ensure that every instance of an `Event` across all `LinkedChunk`s is
+updated when one instance of that `Event` is updated. For efficiency,
+a new index was added to the `EVENTS` object store that tracks the
+`EventId` of an `Event`, so that all instances of an `Event` across
+all `LinkedChunk`s could be retrieved with a single query.