Document daemon-mode ingestion

[funnelfiasco]

This is adjacent to #130 and #140, so it may not fit as separate content, but...

ChainLoop's GUAC guide mentions that ingestion has to be done manually. But in guacsec/guac#1005, we say that "the GCS (and all the other collectors) are already set up to do polling to fetch periodically."

This needs to be made clear in the docs.