Docs for testing locally running websites on Android/iOS real device

[garg3133]

If a user is testing their website, which is running locally on their system, on Android or iOS real device, they can't do so out of the box in Nightwatch, but need to follow some steps:

• Make sure their website is available on their local network.
• Make sure their system and the phone their testing on are connected to the same network.
• Add launch_url property to their Nightwatch conf env being used to run test on Android/iOS real device and use that launch_url inside their tests.

So, we need to have some documentation related to it.

On a side note, while the locally hosted website tests work out of the box for iOS Simulators and support for Android Emulator was added in this PR, the Android PR won't work if users have put the URLs in their tests itself. So, we should somehow suggest them to use launch_url property in Nightwatch config while writing tests for locally hosted websites to be run on mobile devices.