Add development hints to Readme

[Ashark]

I had to find a way for testing this project. I think this info could be added to Readme.

Currently, I have the following reproduce hints.

Windows with error

To invoke an error window, rather quick way is to go to menu "Steam" -> "Change Account...". The new window will appear and you can test if it is managed or not.

Notifications

To invoke a test notification, I know a one way, but not sure it's best. Create a second steam account and send youself a personal message.
It is easier to do via web client, instead of from virtual machine. Create a second user in chromium browser. Then go to https://steamcommunity.com/chat/, login to your second steam account. Now you can quickly create a steam notification by sending a personal message to your first account.