update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/gallery-bank/trunk@873281 b8457f37-d9ea-0310-8a92-e5e31aec5664
update download count

git-svn-id: https://plugins.svn.wordpress.org/gallery-bank/trunk@873282 b8457f37-d9ea-0310-8a92-e5e31aec5664