Document calibnet upgrade process since F3 is already active on that network

[masih]

Calibnet is already running F3 in active more: no passive testing is possible on Calibnet. This makes testing on calibration net limited to just testing backward compatibility.

• Work out the process of upgrading F3 on Calibnet, and investigate any additional steps needed to roll it out on that network gracefully.
• Test the calibnet upgrade scenario on Butterflynet:
  • Leave F3 to activate using the old version.
  • Roll out the new version and assert that F3 progress continues.