Update readme.

git-svn-id: https://plugins.svn.wordpress.org/clean-image-filenames/trunk@1001355 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update short description.

git-svn-id: https://plugins.svn.wordpress.org/clean-image-filenames/trunk@1001357 b8457f37-d9ea-0310-8a92-e5e31aec5664