added SpecialChannelPage with a description for /mentions

src/widgets/settingspages/specialchannelspage.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+#include "widgets/settingspages/settingspage.hpp"
+
+namespace chatterino {
+namespace widgets {
+namespace settingspages {
+class SpecialChannelsPage : public SettingsPage
+{
+public:
+    SpecialChannelsPage();
+};
+}  // namespace settingspages
+}  // namespace widgets
+}  // namespace chatterino