docs: add demo credentials

[PratyushSingh07]

Summary:

Summarize your issue in one sentence (what goes wrong, what did you expect to happen).

Steps to reproduce:

How can we reproduce the issue?

Expected behaviour:

What did you expect the app to do?

Observed behaviour:

What did you see instead? Describe your issue in detail here.

Device and Android version:

What make and model device (e.g., Samsung Galaxy S3) did you encounter this on? What Android
version (e.g., Android 4.0 Ice Cream Sandwich or Android 6.0 Marshmallow) are you running? Is it
the stock
version from the manufacturer or a custom ROM?

Screenshots:

Can be created by pressing the Volume Down and Power Button at the same time on Android 4.0 and higher.