Add MELPA stable badge

Author: zonuexe

README.org

@@ -1,6 +1,7 @@
 * magic-filetype
 #+BEGIN_HTML
 <a href="http://melpa.org/#/magic-filetype"><img alt="MELPA magic-filetype" src="http://melpa.org/packages/magic-filetype-badge.svg"</a>
+<a href="http://stable.melpa.org/#/magic-filetype"><img alt="MELPA stable: magic-filetype" src="http://stable.melpa.org/packages/magic-filetype-badge.svg"></a>
 <a href="https://gitter.im/zonuexe/magic-filetype.el?utm_source=badge&amp;utm_medium=badge&amp;utm_campaign=pr-badge&amp;utm_content=badge"><img alt="Join the chat at https://gitter.im/zonuexe/magic-filetype.el" src="https://badges.gitter.im/Join%20Chat.svg"></a>
 #+END_HTML