doc(event cache): add the copyright notice and basic module doc comment

crates/matrix-sdk-sqlite/src/event_cache_store.rs

@@ -1,3 +1,19 @@
+// Copyright 2024 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! A sqlite-based backend for the [`EventCacheStore`].
+
 #![allow(dead_code)] // Most of the unused code may be used soonish.
 
 use std::{borrow::Cow, fmt, path::Path, sync::Arc};