Add microphone requirement to Stretch AI hardware requirements

[hello-jkoubi]

This PR adds a missing hardware requirement to the README.

When running voice-enabled demos such as:

python -m stretch.app.ai_pickup --use_llm --use_voice

a microphone must be connected to the GPU computer so the system can
capture speech input. Without an available audio input device, the
voice interface fails at runtime.

This update clarifies the requirement to avoid confusion during setup.

[hello-robot-shehab]

Thank you for adding this clarification!