docs+README: add mainnet safety guidelines #1613

Job	Run time
Sqlc check	34s
go mod check	1m 4s
RPC check	1m 49s
test vector check	1m 23s
Compilation check	40s
cross compilation	13m 51s
Docker build check	4m 11s
Lint check	3m 39s
Format check	33s
run itests	9m 17s
run itests postgres	12m 10s
run unit tests (unit-race)	6m 17s
run unit tests (unit-cover)	5m 14s
run unit tests (unit dbbackend=postgres)	6m 58s
	1h 7m 40s

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