Add doc.go for package documentation

doc.go

@@ -0,0 +1,47 @@
+/*
+Package asynq provides a framework for background task processing.
+
+The Client is used to register a task to be processed at the specified time.
+
+	client := asynq.NewClient(&asynq.RedisConfig{
+	    Addr: "localhost:6379",
+	})
+
+	t := &asynq.Task{
+	    Type:    "send_email",
+	    Payload: map[string]interface{}{"recipient_id": 123},
+	}
+
+	err := client.Process(t, time.Now().Add(10 * time.Minute))
+
+The Background is used to run the background processing with a given
+handler with the specified number of workers.
+    bg := asynq.NewBackground(20, &asynq.RedisConfig{
+        Addr: "localhost:6379",
+    })
+
+    bg.Run(handler)
+
+Handler is an interface that implements ProcessTask method that
+takes a Task and return an error. Handler should return nil if
+the processing is successful, otherwise return non-nil error
+so that the task will be retried after some delay.
+
+    type TaskHandler struct {
+        // ...
+    }
+
+    func (h *TaskHandler) ProcessTask(task *asynq.Task) error {
+        switch task.Type {
+        case "send_email":
+            // send email logic
+        case "generate_thumbnail":
+            // generate thumbnail image
+        //...
+        default:
+            return fmt.Errorf("unepected task type %q", task.Type)
+        }
+        return nil
+    }
+*/
+package asynq