doc: add select privilege by default to all users

[telezhnaya]

I added it here as the migration https://github.com/near/near-indexer-for-explorer/blob/master/migrations/2021-06-09-102523_grant_select_on_new_tables/up.sql

But I always forget to run it in all new DBs I'm copy-pasting the code from here It will be easier to fix the source