Add Flutter text to speech demo (#1087)

flutter-examples/how-tts-is-created.md

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+# Introduction
+
+This document describes how the [tts](./tts) folder is created.
+
+
+```bash
+flutter create --platforms windows,macos,linux,android,ios tts
+```
+
+It prints the following:
+
+```
+Developer identity "Apple Development: xxx@zzz.com (xxxxxxx)" selected for iOS code signing
+Creating project tts...
+Resolving dependencies in `tts`... (1.3s)
+Downloading packages...
+Got dependencies in `tts`.
+Wrote 122 files.
+
+All done!
+You can find general documentation for Flutter at: https://docs.flutter.dev/
+Detailed API documentation is available at: https://api.flutter.dev/
+If you prefer video documentation, consider: https://www.youtube.com/c/flutterdev
+
+In order to run your application, type:
+
+  $ cd tts
+  $ flutter run
+
+Your application code is in tts/lib/main.dart.
+```
+
+```
+cd tts
+flutter pub get
+flutter build macos
+flutter run -d macos
+```