message: Update migration guide, add sample for wraping sync into async
diff: @@ -467,6 +467,31 @@ options.Retry.RetryDelay = std::chrono::milliseconds(delta_backoff);
  options.Retry.MaxRetries = maxAttempts;
  ```
  +### Asynchronous API
  +
  +Unfortunately, we don't support asynchronous interface in v12 SDK. You could wrap synchronous functions into asynchronous with some async framework like `std::async`. But note that I/O operations are still performed synchronously under the hood. There's no performance gain with this method.
  +
  +v7.5
  +```C++
  +auto task = blob_client.download_text_async().then([](utility::string_t blob_content) {
  + std::wcout << "blob content:" << blob_content << std::endl;
  +});
  +// Do something else
  +task.wait();
  +```
  +
  +v12
  +```C++
  +auto task = std::async([blobClient]() {
  + auto response = blobClient.Download();
  + std::vector<uint8_t> blobContent = response.Value.BodyStream->ReadToEnd();
  + std::string text(blobContent.begin(), blobContent.end());
  + std::cout << "blob content: " << text << std::endl;
  +});
  +// Do something else
  +task.wait();
  +```
  +
  ## Additional information
  ### Samples
  