[docs] Adding an app icon instructions

[Hackerman342]

Use material design icons and select highest possible resolution
Sized up icons can be blurry, so we want get 80 / 90 pixel icons

[Hackerman342]

Have two options:

1. Quick (use material design icons)
2. Better (use figma and export as 90 px)

[edgarriba]

lets schedule a call with Tamara this week to get the right information about this.