Screenshot url added to readme.txt file

git-svn-id: https://plugins.svn.wordpress.org/mlr-timeline/trunk@813821 b8457f37-d9ea-0310-8a92-e5e31aec5664
Donate link added to readme.txt file

git-svn-id: https://plugins.svn.wordpress.org/mlr-timeline/trunk@813822 b8457f37-d9ea-0310-8a92-e5e31aec5664