removing old readme-file

git-svn-id: https://plugins.svn.wordpress.org/visual-editor-custom-buttons/trunk@820939 b8457f37-d9ea-0310-8a92-e5e31aec5664
adding new readme-file

git-svn-id: https://plugins.svn.wordpress.org/visual-editor-custom-buttons/trunk@820940 b8457f37-d9ea-0310-8a92-e5e31aec5664