Docs: note about pgsql being the default driver

[profnandaa]

This will help those using other DBs other that Postgres, I was getting an error when running migrations.

[PDOException]         
could not find driver

I had to add this line on the .env:

DB_CONNECTION=mysql

[unicodeveloper]

Thanks, someone also had this issue some time back. I'll be super glad if you could raise a PR that states this in the doc @profnandaa

[profnandaa]

Will be on that sir @unicodeveloper