* format place_tracker README
* update sentence in README
* sort Dart members, remove unnecessary type declarations
* Run place_tracker on Android and iOS, update project files
* add link to developer preview caveat
* grammar
* update MAINTENANCE
This PR adds a Flutter sample app that shows an implementation of the "infinite list" UX pattern. That is, a list is shown to the user as if it was continuous although it is internally paginated. This is a common feature of mobile apps, from shopping catalogs through search engines to social media clients.