Updating readme.txt to make security issues more clear.

git-svn-id: https://plugins.svn.wordpress.org/ninja-forms/trunk@990294 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updating Required WordPress version.

git-svn-id: https://plugins.svn.wordpress.org/ninja-forms/trunk@990298 b8457f37-d9ea-0310-8a92-e5e31aec5664