update README; use ISO 8601 timestamps

[sydb]

README

• Fix a few minor typos
• Be more explicit about output directory path
• Mention that it works in R.Pi Bullseye
• Include instruction to set input to pulse when using USB on R.Pi

code

• Use ISO 8601 timestamps both in messages (as date & time) and as timestamp of output .mp3 file (as dateTime).
• Also comment on resetting of vox_silence_time, mostly as a way to ask a question.

Note: I am not a Python programmer, so code should probably be checked and tested kinda carefully.