As the matter of fact, creating new plugin is just few clicks away in the Unreal Engine editor. Still, a lot of people miss it somehow.
To find the plugin creator go into the "Plugin" window and press "Add" button in the top left corner.
After pressing it, you should see the window that allows you to select template for your plugin with a decent description to each.
Once you chose desired template, fill in the plugin name, author and a description and then press "Create Plugin".
That's it, all files for you plugin should be generated and you can use it however you want.