Readme.md documentation

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Issue

Why should i also install the pg module in addition to the @fastify/postgres module?

[Eomm]

Because you may don't want to use the pg module: https://github.com/fastify/fastify-postgres#pg-option

[climba03003]

pg is not direct dependency of this module.
If you don't explicitly install it, it may not exist depends on package manager or command.