Add documentation about SIGNALD_PORT and SIGNALBOOST_PORT to dev setup

While working through the local dev environment instructions, I ran into errors after running make _.setup :

WARNING: The SIGNALD_PORT variable is not set. Defaulting to a blank string.
WARNING: The SIGNALBOOST_PORT variable is not set. Defaulting to a blank string.
ERROR: The Compose file './docker-compose-dev.yml' is invalid because:
services.app.expose is invalid: should be of the format 'PORT[/PROTOCOL]'
services.signald.expose is invalid: should be of the format 'PORT[/PROTOCOL]'

I tried setting these as environment variables and got rid of the warning, but didn't have any luck in resolving the error. Commenting out the expose lines resulted in the make command running successfully.

Is this something that needs to be added to the setup instructions? I'm guessing that team-friendo handles this with the env.gpg file. I'm happy to make the docs pr if y'all can point me in the right direction around what the env var should be set to.