Show description for each city sensor

[vivek32ta]

Resolves #9

I've used Alert.alert() to show descriptions for each sensor using the prop onPress

The descriptions I've used are as follows:

• PM2.5 : This refers to atmospheric particulate matter that has a diameter less than 2.5 micrometers in micrograms per cubic meter.
• Temperature : This is the average temperature in Fahrenheit.
• Humidity : This is the concentration of water vapor present in the air.
• Sensor Count : This is the number of sensors present around this city.
• Population : This is the total population of this city.

Thank you for letting me contribute. I had a good time working on your app.