Add more examples how to use synapse_auto_compressor

the readme only suggests

synapse_auto_compressor -p postgresql://user:pass@localhost/synapse -c 500 -n 100

This means, that it will only check in 100 rooms and quit as I understand is as a layman.

I got the result after 100 chunks:

INFO synapse_auto_compressor::manager] Finished running compressor. Saved 15891 rows. Skipped 4/100 chunks

this seems to have worked.

Then I started again running with `-n 10000` now to try to compress my 56GB database.

Is this the best option? or could you provide more informatinon in the readme please?

Start with the README example for synapse_auto_compressor and the command-line options -p, -c, and -n. Clarify what the room and chunk limits mean, explain whether increasing -n is appropriate for a large PostgreSQL database, and add examples that show how to run the compressor to completion.

Written by the indexing model from the issue text.