Update README.md step 6 to add this.loading$ to constructor

[WhitWaldo]

Step 6 of README.md instructed users to modify the loading property to be an Observable, but then never referred to where to set it from. Updated the constructor to reflect that it comes from the service per documentation at https://github.com/johnpapa/angular-ngrx-data/blob/master/docs/entity-services.md#create-a-custom-entitycollectionservice

[johnpapa]

thanks