edit to readme file with size attribute

git-svn-id: http://plugins.svn.wordpress.org/lbdesign-button-shortcode/trunk@1287343 b8457f37-d9ea-0310-8a92-e5e31aec5664

readme.txt

  how to use:
 
- [button link="http://something.com" color="red"]Call to Action![/button]
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+ [button link="http://something.com" color="red" size="big"]Call to Action![/button]
\ No newline at end of file