Added mm2markdown.xsl template to external-resources folder

Completing request for a pull request started in this discussion:
http://sourceforge.net/apps/phpbb/freeplane/viewtopic.php?f=1&t=235&sid=a91d0ebd13747463e1dd06046fc2ff72&start=10#wrap

The mm2markdown.xsl template adds "Export to Markdown" functionality.