Add README

Author: YuryHrytsuk

services/postgres/README.md

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+## Postgres configuration
+
+Read and follow instructons in `./scripts/init.sql` script. This needs to be executed once in every postgres database we run (both self-hosted and RDS)
+
+Create role and users scripts needs to be run on demand (e.g. in case we need a readonly user). Generate scripts using repo config values, read and follow instructions inside. This needs to be executed once.