Added covid-19 example app and Updated the readme

[mehdi1514]

Added an app that tracks Coronavirus cases. It uses HTTP requests to send requests to NovelCOVID API and receive responses from there. It also includes a visualization of the received data.

The code is split into different files. The hierarchy is given below:

lib
    - resources
        - fetch_data_functions.dart
    - screens
        - countrylist.dart
        - home.dart
        - searchList.dart
    - widgets
        - drawer.dart
        - foldable_cell_widgets.dart
        - graph.dart
        - info_card.dart

I added comments wherever I felt I should so that it is easy to understand. The package name is github.nisrulz.covid19mobileapp as mentioned in the guidelines. The code is also formatted(with Reformat Code feature of vs code).

The Flutter version used in this projects is v 1.22.0(latest)