Add example of handling a required argument

More from:

    https://github.com/twitchtv/twirp/wiki/Best-Practices

hwt.go

@@ -4,11 +4,15 @@ import (
 	"context"
 	fmt "fmt"
 
+	"github.com/twitchtv/twirp"
 	pb "mcquay.me/hwt/rpc/hwt"
 )
 
 type Server struct{}
 
 func (s *Server) Hello(ctx context.Context, req *pb.HelloReq) (*pb.HelloResp, error) {
+	if req.Subject == "" {
+		return nil, twirp.RequiredArgumentError("subject")
+	}
 	return &pb.HelloResp{Text: fmt.Sprintf("echo: %v", req.Subject)}, nil
 }

rpc/hwt/service.proto

@@ -9,9 +9,9 @@ service HelloWorld {
 }
 
 message HelloReq {
-  string subject = 1;
+  string subject = 1; // required
 }
 
 message HelloResp {
-  string text = 1;
+  string text = 1; // required
 }