TextEditor: Add help documentation

Author: https://github.com/bcoles
Commit: https://github.com/SerenityOS/serenity/commit/aa8cb35b908
Pull-request: https://github.com/SerenityOS/serenity/pull/4975

Base/usr/share/man/man1/TextEditor.md

@@ -0,0 +1,28 @@
+## Name
+
+TextEditor - SerenityOS text editor
+
+## Synopsis
+
+```**sh
+$ TextEditor [--preview-mode mode] [file]
+```
+
+## Description
+
+TextEditor is a text document editor for SerenityOS featuring a preview mode
+which allows automatic live rendering of HTML and Markdown documents.
+
+## Options
+
+* `--preview-mode mode`: Preview mode, one of 'none', 'html', 'markdown', 'auto'
+
+## Arguments:
+
+* file: File to edit
+
+## Examples
+
+```sh
+$ TextEditor /home/anon/Documents/emoji.txt
+```

Userland/Applications/TextEditor/TextEditorWidget.cpp

@@ -488,6 +488,9 @@ TextEditorWidget::TextEditorWidget()
     syntax_menu.add_action(*m_shell_highlight);
 
     auto& help_menu = menubar->add_menu("Help");
+    help_menu.add_action(GUI::CommonActions::make_help_action([](auto&) {
+        Desktop::Launcher::open(URL::create_with_file_protocol("/usr/share/man/man1/TextEditor.md"), "/bin/Help");
+    }));
     help_menu.add_action(GUI::CommonActions::make_about_action("Text Editor", GUI::Icon::default_icon("app-text-editor"), window()));
 
     GUI::Application::the()->set_menubar(move(menubar));