Updated README to run the example.

[manhluong]

Why this PR?

Executing flutter run in the example folder, with a running iPhone XR simulator, causes the following error:

Launching lib/main.dart on iPhone XR in debug mode...

Oops; flutter has exited unexpectedly.
Sending crash report to Google.

From the log, I can read:

FileSystemException: FileSystemException: Getting current working directory failed, path = '' (OS Error: Too many open files, errno = 24)

Which is the problem reported here: trentpiercy/trace#1

How this PR fixes the issue?

Updated the README with the instructions I found in the link.

What are the expected outcomes after the merge?

It is possible to run the example app in the iOS simulator.

[johnpryan]

thanks! I think this is happening because the example uses a lot of tiles... added an issue to track #269