Added MSDN links

Author: zsxwing

rxjava-core/src/main/java/rx/Observable.java

@@ -542,6 +542,7 @@ public static <T> Observable<T> create(OnSubscribeFunc<T> func) {
      *            the type of the items (ostensibly) emitted by the Observable
      * @return an Observable that returns no data to the {@link Observer} and immediately invokes
      *         the {@link Observer}'s {@link Observer#onCompleted() onCompleted} method
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229670(v=vs.103).aspx">MSDN: Observable.Empty Method</a>
      */
     public static <T> Observable<T> empty() {
         return from(new ArrayList<T>());
@@ -559,6 +560,7 @@ public static <T> Observable<T> empty() {
      * @return an Observable that returns no data to the {@link Observer} and immediately invokes
      *         the {@link Observer}'s {@link Observer#onCompleted() onCompleted} method with
      *         the specified scheduler.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229066(v=vs.103).aspx">MSDN: Observable.Empty Method (IScheduler)</a>
      */
     public static <T> Observable<T> empty(Scheduler scheduler) {
         return Observable.<T>empty().subscribeOn(scheduler);