Add FAQ about browser verbosity levels to prevent questions about debug method not working

Every other methods: `error, warn, info, log`  work fine and print the value, but `debug` prints nothing and no error message to be found.

Using chrome `Version 59.0.3071.115 (Official Build) (64-bit)`

I use in app.module:
```
import { Logger, Options as LoggerOptions, Level as LoggerLevel } from "angular2-logger/core";

@NgModule({
  providers: [
    { provide: LoggerOptions, useValue: { level: 5 } },
    Logger
  ]
})
export class AppModule { }
```

In the service:
```
import { Logger } from "angular2-logger/core";

@Injectable()
export class MyService {

  constructor(private logger: Logger) {
    this.logger.debug("Debug message...")
  }
}
```

Description