doc: Database requirements

[tarlepp]

There should be some requirements in docs to tell people that they need database that supports JSON type. Noticed this one when fixing TravisCI build process within #12

The error that occurs within database that doesn't support JSON type is shown here

[JoppeDC]

@tarlepp Turns out this is one of the requirements for Sylius itself. Theoretically my plugin supports it since no JSON type is used. Maybe its better if we link to them. Sylius Requirements