Extensions Readme is a Visual Studio Code extension that lists all extensions installed in order to show its readme files so you won't need to search it on the extensions marketplace tree/sidebar.
F1 then type extensions list it will show command Extensions: List them... hit Enter and that will list all non builtin extensions, choose one and it will open its readme markdown file.
You can always assing a hotkey with Visual Studio Code's builtin hotkeys manager.
You can also click the menu in the top bar (icons) to open its remote repository, marketplace, local changelog file and local source file.