Add french language in readme

git-svn-id: https://plugins.svn.wordpress.org/img-lazy-loading/trunk@1055332 b8457f37-d9ea-0310-8a92-e5e31aec5664
change readme.txt

git-svn-id: https://plugins.svn.wordpress.org/img-lazy-loading/trunk@1055334 b8457f37-d9ea-0310-8a92-e5e31aec5664