Add pro tips on handling different versions of ModularSensors

[aufdenkampe]

including both platformio.ini file changers and also upgrading station code sketches from one version to another.

Here's some example content pasted from a recent email.

Upgrade by changing your PlatformIO.ini file

• Under lib_deps = , comment out EnviroDIY_ModularSensors@=0.21.2, and replace with any of the below
  • EnviroDIY_ModularSensors@=0.21.4​ // for any of the official releases at https://github.com/EnviroDIY/ModularSensors/releases​, or
  • https://github.com/EnviroDIY/ModularSensors.git#​develop // to point to the head of any branch, or
  • https://github.com/EnviroDIY/ModularSensors.git#​bf667dc33ef7e48a38fd55a9adc9579331c344c4 // to point to any specific commit
• For documentation on the syntax options, see https://docs.platformio.org/en/latest/userguide/lib/cmd_install.html#platformio-lib-install