Add meaning of toggle options to README.md

[Francewhoa]

Hi there :)

I love "gnome-shell-extension-suspend-button". It works with Debian 8 Jessie with GNOME 3.14. It is easy to use. Lightweight. More secure because its code is publicly available for review on GitHub.

I suggest to add meaning of toggle options to the README.md file. I was wondering about their meaning. I searched both the present README.md file and the https://extensions.gnome.org/extension/826/suspend-button/ page but found no answer. Then I found that moving the mouse over each option, wait a few seconds will display a description. That's useful but not obvious. I suggest to keep that but I suggest to also add that information to the README.md file. It would be useful.

I'm contributing the following draft to add to the README.md file. Feel free to adapt it to your linking. It's inspired from the popup help.

The "Two buttons" toggle option when ON display both a suspend button and a shutdown button. When OFF it display only a Suspend button. 

The "Suspend default" toggle option when ON change alt-switch-buttons behaviour to default to suspend. Press alt to shutdown. This setting has no effect if "Two buttons" toggle is set to ON.

Cheers,

Francewhoa

[laserb]

Thank you for the feedback. Adding some more information to the readme is a good idea.