Update addons.markdown

Updates addons.markdown to mention AtExit() function.  Addresses issue #8324.
@sam-github the documentation is pulled from the [text of the commit](https://github.com/joyent/node/commit/e4a8d2617b49727c4de09ad9ca9bc8fa270adf1e) that introduces the function -- I think it runs right before the process ends.