add screenshot,change readme.txt.

git-svn-id: https://plugins.svn.wordpress.org/wp-hash-filename/trunk@1012843 b8457f37-d9ea-0310-8a92-e5e31aec5664
change txt code.

git-svn-id: https://plugins.svn.wordpress.org/wp-hash-filename/trunk@1012846 b8457f37-d9ea-0310-8a92-e5e31aec5664