Document the `client.observe()` API.

Resolves #148.

Signed-off-by: Tao He <sighingnow@gmail.com>

README.md

@@ -863,14 +863,31 @@ pplx::task<Response> proclaim(std::string const &name, int64_t lease_id,
 
 pplx::task<Response> leader(std::string const &name);
 
-std::unique_ptr<Observer> observe(std::string const &name,
-                                  std::function<void(Response)> callback,
-                                  const bool once = false);
+std::unique_ptr<Observer> observe(std::string const &name);
 
 pplx::task<Response> resign(std::string const &name, int64_t lease_id,
                             std::string const &key, int64_t revision);
 ```
 
+The `Observer` returned by `observe()` can be use to monitor the changes of election ownership.
+The observer stream will be canceled when been destructed.
+
+```c++
+  std::unique_ptr<etcd::Observer> observer = etcd.observe("test");
+
+  // wait one change event, blocked execution
+  etcd::Response resp = observer->WaitOnce();
+
+  // wait many change events, blocked execution
+  for (size_t i = 0; i < ...; ++i) {
+    etcd::Response resp = observer->WaitOnce();
+    ...
+  }
+
+  // cancel the observer
+  observer.reset(nullptr);
+```
+
 for more details, please refer to [etcd/Client.hpp](./etcd/Client.hpp).
 
 ### TODO

etcd/SyncClient.hpp

@@ -642,7 +642,7 @@ namespace etcd
     class Observer {
       public:
         ~Observer();
-        // wait a at least one response from the observer.
+        // wait at least *one* response from the observer.
         Response WaitOnce();
       private:
         std::shared_ptr<etcdv3::AsyncObserveAction> action = nullptr;