Add documentation for testing 'could not contact integrations server' error

[turt2live]

Requires:

• Widget for testing riot <--> integration manager <--> homeserver auth #244
• Add a command for creating custom widgets without an integration manager matrix-org/matrix-react-sdk#2824 (or similar)
• Widget OpenID reauth implementation matrix-org/matrix-react-sdk#2781

Expected documentation would be: Create a room and type /addwidget https://dimension.t2bot.io/widgets/manager-test then click the button. If everything comes out green and you're still seeing the error, verify that your client has picked up the configuration changes.