Add README on adding new read-only accounts to PostgreSQL

[frol]

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[frol]

Unfortunately, the instructions we used (and provided in the PR) are not sufficiently strict, and allow creating new tables, inserting values there, creating new materialized views etc.

Thanks @telezhnaya for reporting it. @telezhnaya May I ask you to find the right set of incantations to achieve the true read-only mode? 😄

I have seen a few places where it is explained that even a new user with no permission granted has access to the public schema sufficient to create new tables (and thus owning those), and they suggest revoking permissions to public, but I was not very successful. Some references:

• https://gist.github.com/oinopion/4a207726edba8b99fd0be31cb28124d0
• https://stackoverflow.com/questions/48663891/postgresql-readonly-role-and-user (it would be ideal to create a readonly_access role)

[telezhnaya]

it is explained that even a new user with no permission granted has access to the public schema sufficient to create new tables

That's true. That's why it's better to use non-default schema for the project.
public is not just regular schema, it has some exceptions as the default option.

Ideally, we need to invoke this command from admin account: REVOKE CREATE ON SCHEMA public FROM PUBLIC;
It's enough to invoke this command once, and all users except admin will have read-only access to the tables. With that, we will not need to edit our instruction.

My commit was tested it only in the sandbox. I need another account (with more permissions) to test it in real DB.

[telezhnaya]

To sum up:
We have new migration, we also need to invoke

REVOKE CREATE ON SCHEMA PUBLIC FROM PUBLIC;
REVOKE ALL PRIVILEGES ON ALL TABLES IN SCHEMA PUBLIC FROM PUBLIC;

In my opinion, it should not be a part of migration

[telezhnaya]

@frol I deployed these changes to mainnet and testnet

[telezhnaya]

#89