add docs for health checks (#599)

examples/src/health/README.md

@@ -0,0 +1,16 @@
+# Health checks
+
+gRPC has a [health checking protocol](https://github.com/grpc/grpc/blob/master/doc/health-checking.md) that defines how health checks for services should be carried out. Tonic supports this protocol with the optional [tonic health crate](https://docs.rs/tonic-health).
+
+This example uses the crate to set up a HealthServer that will run alongside the application service. In order to test it, you may use community tools like [grpc_health_probe](https://github.com/grpc-ecosystem/grpc-health-probe).
+
+For example, running the following bash script:
+
+```bash
+while [ true ]; do
+./grpc_health_probe -addr=[::1]:50051 -service=helloworld.Greeter
+sleep 1
+done
+```
+
+will show the change in health status of the service over time.