Add example for access keys in menus and feature detection (#390)

menu-accesskey-visible/README.md

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+# menu-accesskey-visible
+
+Demonstrates access keys in the [menus API](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/menus/).
+
+**This add-on uses the `menus` namespace to access the functions it needs to create menu items. Note that Chrome, Edge, and Opera all use the `contextMenus` namespace for this, so this extension will not work in these browsers. For compatibility with these browsers, Firefox also offers the `contextMenus` namespace, so to make this extension work with other browsers, use `contextMenus`.**
+
+## What it does
+
+This extension adds a menu item that has an access key.
+An access key allows users to activate the menu item with one keystroke.
+On Linux and Windows, the chosen access key is underlined.
+On macOS, access keys are supported too, but without visual indicators.
+
+When an access key is shared by multiple menu items, pressing the key
+will switch between the menu items that share the access key. Otherwise
+(if an access key is bound to one menu item only), the menu item is clicked.
+
+The menu item in this demo logs a message to the extension's console upon
+click. This console can be shown via the "Debug" button at `about:debugging` .
+
+## What it shows
+
+* Specifying access keys via `&` in the menu item title.
+* Detecting feature support, exemplified by detecting support for the `visible`
+  property of the `menus.update` API that was also introduced in Firefox 63.
+
+As access keys are only supported in Firefox 63 and later, this example also
+shows how one can detect the feature support in the browser and fall back to
+menu items without access keys if needed.