docs: hammerjs using systemjs.config

guides/getting-started.md

@@ -189,6 +189,12 @@ System.config({
     '@angular/cdk/platform': 'npm:@angular/cdk/bundles/cdk-platform.umd.js',
     '@angular/cdk/a11y': 'npm:@angular/cdk/bundles/cdk-a11y.umd.js',
     // ...
+    'hammerjs': 'npm:hammerjs',
+  },
+  packages: {
+    //...
+    hammerjs: {main: './hammer.min.js', defaultExtension: 'js'}
+    //...
   }
 });
 ```