Document how to use the http interceptor

Author: mpalourdio

README.MD

@@ -12,6 +12,8 @@ $ npm install ng-http-loader --save
 This package provides an HTTP Interceptor, and a spinner component. The HTTP interceptor listens to all HTTP requests
 and shows a spinner during pending requests.
 
+In order to have this module listening to your http requests, replace all your ``http`` uses by [http: HttpInterceptorService](src/app/http-interceptor.service.ts).
+
 It's compatible with Angular 4+.
 
 The spinner has been taken from [SpinKit](https://github.com/tobiasahlin/SpinKit).