readme: add usage section

README.md

@@ -7,3 +7,85 @@
 See [docs and examples on pkg.go.dev](https://pkg.go.dev/github.com/soroushj/grpcmock).
 
 See the [example](./example) directory for a complete example.
+
+## Usage
+
+```go
+import (
+	"context"
+	"log"
+	"net"
+
+	"github.com/soroushj/grpcmock"
+	"github.com/soroushj/grpcmock/example/notes"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+func Example() {
+	// Create a gRPC server using a grpcmock interceptor
+	mock := grpcmock.New()
+	server := grpc.NewServer(grpc.UnaryInterceptor(mock.UnaryServerInterceptor()))
+
+	// Register an implementation of your server; the example Notes server in this case.
+	// This typically should be the generated Unimplemented implementation.
+	notes.RegisterNotesServer(server, &notes.UnimplementedNotesServer{})
+
+	// Run the server on an available port
+	lis, err := net.Listen("tcp", ":0")
+	if err != nil {
+		log.Fatalf("listen: %v", err)
+	}
+	go func() {
+		if err := server.Serve(lis); err != nil {
+			log.Fatalf("serve at %v: %v", lis.Addr(), err)
+		}
+	}()
+
+	// At this point, if you call any method from the running server, you will get an Unimplemented error.
+	// Let's change this behavior for the GetNote method of this server.
+
+	// This is how you can set a mock response for a method; an error in this case.
+	// After this, you will get a NotFound error from GetNote instead of an Unimplemented error.
+	mock.SetResponse("GetNote", &grpcmock.UnaryResponse{
+		Err: status.Error(codes.NotFound, "note not found"),
+	})
+
+	// Similarly, you can set a non-error response.
+	// After this, you will get the response below from GetNote instead of an error.
+	mock.SetResponse("GetNote", &grpcmock.UnaryResponse{
+		Resp: &notes.GetNoteResponse{
+			Note: &notes.Note{
+				Id:   "1",
+				Text: "a",
+			},
+		},
+	})
+
+	// If you need something more than a simple response, you can set a handler.
+	// After this, any call to GetNote will be handled using the function below.
+	mock.SetHandler("GetNote", func(ctx context.Context, req interface{}) (interface{}, error) {
+		r := req.(*notes.GetNoteRequest)
+		if r.Id == "1" {
+			return &notes.GetNoteResponse{
+				Note: &notes.Note{
+					Id:   "1",
+					Text: "a",
+				},
+			}, nil
+		}
+		return nil, status.Error(codes.NotFound, "note not found")
+	})
+
+	// You can remove any previously-set response or handler for a method.
+	// After this, GetNote will return an Unimplemented error.
+	mock.Unset("GetNote")
+
+	// You can also remove any response or handler for all methods
+	mock.Clear()
+
+	// Stop the server
+	server.Stop()
+}
+```