Firestore lite overview (#4861)

* Adding some explanation of Firestore Lite.

* Attempting to merge this back to normal state.

* Fixing typo.

* Update index.ts

Co-authored-by: Sebastian Schmidt <[email protected]>

Author: egilmorez

packages/firestore/lite/index.ts

@@ -1,3 +1,12 @@
+/**
+ * Firestore Lite
+ *
+ * @remarks Firestore Lite is a small online-only SDK that allows read
+ * and write access to your Firestore database. All operations connect
+ * directly to the backend, and `onSnapshot()` APIs are not supported.
+ * @packageDocumentation
+ */
+
 /**
  * @license
  * Copyright 2020 Google LLC