README.md: explain linera-client (#2573)

README.md

@@ -45,6 +45,11 @@ from low to high levels in the dependency graph)
   Defines the data-type for RPC messages (currently all client ↔ proxy ↔
   chain ↔ chain interactions), and track the corresponding data schemas.
 
+* [`linera-client`](https://linera-io.github.io/linera-protocol/linera_client/index.html)
+  Library for writing Linera clients.  Used for the command-line
+  client and the node service in `linera-service`, as well as the Web
+  client in [`linera-web`](https://github.com/linera-io/linera-web/).
+
 * [`linera-service`](https://linera-io.github.io/linera-protocol/linera_service/index.html)
   Executable for clients (aka CLI wallets), proxy (aka validator frontend) and servers.