ZcashClient.sync()

Currently synchronous, but that's fine for a demo.

README.md

@@ -12,7 +12,26 @@
 $ ./build.sh
 ```
 
-## Running
+## Running the backend
+
+Web browsers currently cannot talk directly to gRPC servers, so it is necessary to run a
+proxy as part of the backend. The `envoy/` subdirectory contains a Dockerfile and config
+file for an Envoy proxy that listens on `localhost:8081` and will route requests to a
+`lightwalletd` frontend listening on `localhost:9067`.
+
+See [the `lightwalletd` documentation](https://github.com/zcash-hackworks/lightwalletd)
+for details on how to set up a local `lightwalletd` testnet instance. Note that when
+starting the frontend, you may need to use `--bind-addr 0.0.0.0:9067` so that the Docker
+container can access it.
+
+To build and run the Envoy proxy:
+
+```sh
+$ docker build -t lightwalletd/envoy -f envoy/envoy.Dockerfile envoy
+$ docker run -d -p 8081:8081 --network=host lightwalletd/envoy
+```
+
+## Running the demo
 
 ```sh
 $ cd demo-www