Add MySQL setup guide for run & testing in README

[martiagop]

Description

For new developers, who wish to test or contribute to the GTA:Open project, it is important to have an initial setup guide for the MySQL database.

This should include the required environment (MySQL), the order of creation (or import) of the .SQL files, and the steps necessary to connect to the database (mysql.ini).

Additional information/screenshots

While reading the code, I thought it uses environment variables (.env), but when I tried, using (mysql.ini) it was accepted as well. If possible, specify both scenarios.