docs+README: add mainnet safety guidelines #1610

Job	Run time
Compilation check	40s
cross compilation	13m 38s
Docker build check	4m 6s
Format check	36s
run itests	8m 7s
run itests postgres	10m 56s
Lint check	4m 9s
go mod check	1m 9s
RPC check	1m 30s
Sqlc check	34s
test vector check	1m 30s
run unit tests (unit-race)	7m 19s
run unit tests (unit-cover)	14m 17s
run unit tests (unit dbbackend=postgres)	5m 12s
	1h 13m 43s

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