Use upload instead of URL

git-svn-id: https://plugins.svn.wordpress.org/kraken-image-optimizer/trunk@976222 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated readme.txt

git-svn-id: https://plugins.svn.wordpress.org/kraken-image-optimizer/trunk@976224 b8457f37-d9ea-0310-8a92-e5e31aec5664