v3.x give write permission to storage

[scips]

I'm not sure if it should be in the doc in the v3.x installation section or in the composer setup part.

But I think it's worth to mention that the storage directory must be writable by the users that is used by the web server (www-data or similar).

Let me know what you think about so I can transform this in pull request.

[jbrooksuk]

I think we should make a point of this, just to be clear on the expectations and requirements when installing Cachet. If you can make a PR, I'd love that!