update readme and main plugin file ready for next release

git-svn-id: https://plugins.svn.wordpress.org/svg-support/trunk@953760 b8457f37-d9ea-0310-8a92-e5e31aec5664
trunk readme - stable tag 2.1.2 for release

git-svn-id: https://plugins.svn.wordpress.org/svg-support/trunk@953767 b8457f37-d9ea-0310-8a92-e5e31aec5664