Clarify operator modes in README

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Benchmarks

metric	old	new	pct change
recall (after create)	0.740	0.740	-
recall (after insert)	0.790	0.758	-4.05%
select bulk tps	494.205	492.178	-0.41%
select bulk latency (ms)	15.518	15.563	+0.29%
select bulk latency (stddev ms)	2.164	2.506	+15.80%
create latency (ms)	1194.806	1198.474	+0.31%
insert bulk tps	11.231	11.378	+1.31%
insert bulk latency (ms)	89.027	87.878	-1.29%
insert bulk latency (stddev ms)	4.080	2.691	-34.04%
disk usage (bytes)	6348800.000	6348800.000	-