Add Documentation link in Help menu

Because the documentation link in 'About Chewing editor is not easy
to be found, add the documentation link in Help menu make user easier
to use

src/ui/ChewingEditor.ui

@@ -92,6 +92,7 @@
     </property>
     <addaction name="actionAbout"/>
     <addaction name="actionAboutQt"/>
+    <addaction name="actionDocumentation"/>
    </widget>
    <addaction name="menuFile"/>
    <addaction name="menuHelp"/>
@@ -122,6 +123,14 @@
     <enum>QAction::AboutQtRole</enum>
    </property>
   </action>
+  <action name="actionDocumentation">
+    <property name="text">
+     <string>Documentation</string>
+    </property>
+    <property name="menuRole">
+     <enum>QAction::ApplicationSpecificRole</enum>
+    </property>
+  </action>
   <action name="actionExit">
    <property name="text">
     <string>E&amp;xit</string>

src/view/ChewingEditor.cpp

@@ -24,6 +24,7 @@
 #include <QtGui>
 #include <QDebug>
 #include <QMessageBox>
+#include <QDesktopServices>
 
 #include "ChewingImporter.h"
 #include "ChewingExporter.h"
@@ -191,6 +192,10 @@ void ChewingEditor::showAbout()
     aboutBox.exec();
 }
 
+void ChewingEditor::showDocumentation(){
+    QDesktopServices::openUrl(QUrl("https://github.com/chewing/chewing-editor/wiki"));
+}
+
 void ChewingEditor::showDeleteConfirmWindow()
 {
     QString text = tr("Do you want to delete this phrase?");
@@ -355,4 +360,7 @@ void ChewingEditor::setupAboutWidget()
    connect(
         ui_.get()->actionAbout, SIGNAL(triggered()), this, SLOT(showAbout())
    );
+   connect(
+        ui_.get()->actionDocumentation, SIGNAL(triggered()), this, SLOT(showDocumentation())
+   );
 }

src/view/ChewingEditor.h

@@ -48,6 +48,7 @@ public slots:
     void selectExportFile();
     void finishFileSelection(const QString& file);
     void showAbout();
+    void showDocumentation();
     void showDeleteConfirmWindow();
 
 private: