Beta - Strapi v4: README updates

[jaskipper]

The README for the beta for Strapi v4 needs updating.

The "How to customize editor" is not functioning at this time. We need to figure out how to do this with the new setup and then update the documentation.

The Link tool thumbnails are being blocked by Strapi's content security policy. The workaround at this time is to replace the 'strapi::security' line in ./config/middlewares.js to the following (do this at your own risk).

module.exports = [
  // ...
  {
    name: 'strapi::security',
    config: {
      contentSecurityPolicy: {
        directives: {
          'img-src': ['*'],
        },
      }
    },
  },
  // ...
];

I'm not certain if there is a way we can override this directly from the plugin or not.

At this time, it's not necessary to give Public and Authenticated role access to the plugin API. ./server/routes/index.js is configured to auth: false. I don't know if this is a security issue or not. Should we leave it up to the user to give Public and Authenticated role access, or can we leave it with auth: false?