Adding some explanation of Firestore Lite.

Author: egilmorez

packages/firestore/lite/index.ts

@@ -1,3 +1,13 @@
+/**
+ * Firestore Lite
+ *
+ * @remarks This package includes the same feature set as Firestore, but
+ * without latency compensation or snapshot listeners (and with
+ * correspondingly lighter size footprint). Firestore Lite supports fast
+ * write and read, operations but only when online.
+ * @packageDocumentation
+ */
+
 /**
  * @license
  * Copyright 2020 Google LLC