Better example code for google protos (#542)

* Better example code for google protos

* Try fix windows build

* Try fix windows build

* Remove generated code in repo

tonic-build/README.md

@@ -21,6 +21,7 @@ tonic-build = <tonic-version>
 
 ### Simple
 
+In `build.rs`:
 ```rust
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     tonic_build::compile_protos("proto/service.proto")?;
@@ -41,3 +42,63 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
    Ok(())
 }
 ```
+See [more examples here](https://github.com/hyperium/tonic/tree/master/examples)
+
+### Google APIs example
+A good way to use Google API is probably using git submodules.
+
+So suppose in our `proto` folder we do:
+```
+git submodule add https://github.com/googleapis/googleapis
+```
+
+And a bunch of Google proto files in structure will be like this:
+```
+├── googleapis
+│   └── google
+│       ├── api
+│       │   ├── annotations.proto
+│       │   ├── client.proto
+│       │   ├── field_behavior.proto
+│       │   ├── http.proto
+│       │   └── resource.proto
+│       └── pubsub
+│           └── v1
+│               ├── pubsub.proto
+│               └── schema.proto
+```
+
+Then we can generate Rust code via this setup in our `build.rs`
+```rust
+fn main() {
+    tonic_build::configure()
+        .build_server(false)
+        //.out_dir("src/google")  // you can change the generated code's location
+        .compile(
+            &["proto/googleapis/google/pubsub/v1/pubsub.proto"],
+            &["proto/googleapis"], // specify the root location to search proto dependencies
+        ).unwrap();
+}
+```
+
+Then you can reference the generated Rust like this this in your code:
+```rust
+pub mod api {
+    tonic::include_proto!("google.pubsub.v1");
+}
+use api::{publisher_client::PublisherClient, ListTopicsRequest};
+```
+
+Or if you want to save the generated code in your own code base,
+you can uncomment the line `.out_dir(...)` above, and in your lib file
+config a mod like this:
+```rust
+pub mod google {
+    #[path = ""]
+    pub mod pubsub {
+        #[path = "google.pubsub.v1.rs"]
+        pub mod v1;
+    }
+}
+```
+See [the example here](https://github.com/hyperium/tonic/tree/master/examples/src/gcp)