Add note about brown-out issues resulting in errors

[benlhy]

Is your feature request related to a problem? Please describe.
If you are experiencing errors in transactions, it could be that the NoteCarrier is insufficiently powered during transmission events, which causes the Notecard to brown out and reset the connection. For I2C this results in a [Errno: 19] error, and for UART this results in an empty JSON response from the Notecard.

Describe the solution you'd like
Add a section that describes potential errors and suggests checking the power supply to the notecard. For me, connecting a LiPo battery instead of just powering off directly from the USB solved this issue. Connecting a sufficiently large capacitor may also help to regulate the power supply.