Docs: Databases not on ".firebaseio.com"

Early in the development of Firebase, all databases were accessible at the URL: 
    https://<projectId>.firebaseio.com

However, now there are multiple locations and instances of databases, and that assumption no longer holds. 
If your database is not at `https://<projectId>.firebaseio.com`, cypress-firebase will seem to hang, rather than giving an explicit error.
For that reason it is very helpful to warn people up front, to put in the URL.

README.md

@@ -49,9 +49,13 @@ If you are interested in what drove the need for this checkout [the why section]
        // NOTE: Add "supportFile" setting if separate location is used
        setupNodeEvents(on, config) {
          // e2e testing node events setup code
-         return cypressFirebasePlugin(on, config, admin);
-         // NOTE: If not setting GCLOUD_PROJECT env variable, project can be set like so:
-         // return cypressFirebasePlugin(on, config, admin, { projectId: 'some-project' });
+         return cypressFirebasePlugin(on, config, admin,{
+             // Here is where you can pass special options. 
+             // If you have not set the GCLOUD_PROJECT environment variable, give the projectId here, like so:
+             //    projectId: 'some-project',
+             // if your databaseURL is not just your projectId plus ".firebaseio.com", then you _must_ give it here, like so:
+             //    databaseURL: 'some-project-default-rtdb.europe-west1.firebasedatabase.app',
+         });
        },
      },
    });
@@ -70,9 +74,13 @@ If you are interested in what drove the need for this checkout [the why section]
        // NOTE: Make supportFile exists if separate location is provided
        setupNodeEvents(on, config) {
          // e2e testing node events setup code
-         return cypressFirebasePlugin(on, config, admin);
-         // NOTE: If not setting GCLOUD_PROJECT env variable, project can be set like so:
-         // return cypressFirebasePlugin(on, config, admin, { projectId: 'some-project' });
+         return cypressFirebasePlugin(on, config, admin,{
+             // Here is where you can pass special options. 
+             // If you have not set the GCLOUD_PROJECT environment variable, give the projectId here, like so:
+             //    projectId: 'some-project',
+             // if your databaseURL is not just your projectId plus ".firebaseio.com", then you _must_ give it here, like so:
+             //    databaseURL: 'some-project-default-rtdb.europe-west1.firebasedatabase.app',
+         });
        },
      },
    });