Add QPromise<T>::timeout documentation

docs/SUMMARY.md

@@ -12,6 +12,7 @@
   * [.isRejected](qtpromise/qpromise/isrejected.md)
   * [.tap](qtpromise/qpromise/tap.md)
   * [.then](qtpromise/qpromise/then.md)
+  * [.timeout](qtpromise/qpromise/timeout.md)
   * [.wait](qtpromise/qpromise/wait.md)
   * [::all (static)](qtpromise/qpromise/all.md)
   * [::reject (static)](qtpromise/qpromise/reject.md)

docs/qtpromise/api-reference.md

@@ -11,6 +11,7 @@
 * [`QPromise<T>::isRejected`](qpromise/isrejected.md)
 * [`QPromise<T>::tap`](qpromise/tap.md)
 * [`QPromise<T>::then`](qpromise/then.md)
+* [`QPromise<T>::timeout`](qpromise/timeout.md)
 * [`QPromise<T>::wait`](qpromise/wait.md)
 
 ### Public Static Members

docs/qtpromise/qpromise/timeout.md

@@ -0,0 +1,18 @@
+## `QPromise<T>::timeout`
+
+```
+QPromise<T>::timeout(int msec, any error = QPromiseTimeoutException) -> QPromise<T>
+```
+
+This method returns a promise that will be resolved with the `input` promise's fulfillment value or rejection reason. However, if the `input` promise is not fulfilled or rejected within `msec` milliseconds, the `output` promise is rejected with `error` as the reason (`QPromiseTimeoutException` by default).
+
+```cpp
+QPromise<int> input = {...}
+auto output = input.timeout(2000)
+    .then([](int res) {
+        // operation succeeded within 2 seconds
+    })
+    .fail([](const QPromiseTimeoutException& e) {
+        // operation timed out!
+    });
+```