Add known issue about USB-C charging failures

Both Birch and Chestnut are affected by issues that cause 12V negotiation and rapid charging to fail.

USB 2.0 could also fail on Birch units.

To help prevent this, follow these step to charge the phone:

• Unplug the power adapter (wall wart) from the power outlet
• Unplug the USB-C cable from both the phone and the adapter
• Plug one end of the USB-cable into the adapter and the other end into the phone
• Plug the adapter into the power outlet

When unplugging:

• Remove the adapter from the power outlet first
• Then unplug the USB-C cable from the phone

5V charging via a USB port on a workstation is unaffected.