docs(aio): add Observable and Rx docs (#21423)

PR Close #21423

aio/content/examples/rx-library/src/naming-convention.ts

@@ -0,0 +1,20 @@
+
+
+import { Component } from '@angular/core';
+import { Observable } from 'rxjs/Observable';
+
+@Component({
+  selector: 'app-stopwatch',
+  templateUrl: './stopwatch.component.html'
+})
+export class StopwatchComponent {
+
+  stopwatchValue: number;
+  stopwatchValue$: Observable<number>;
+
+  start() {
+    this.stopwatchValue$.subscribe(num =>
+      this.stopwatchValue = num
+    );
+  }
+}