Based on the TLP graduation discussion on general@incubator, restore LICENSE.txt from r979180 and rename to LICENSE. Note that the contents of this file should match with the one that Apache's Maven release process and its remote resources bundle would otherwise bring in to the source distro.

git-svn-id: https://svn.apache.org/repos/asf/incubator/shiro/trunk@986550 13f79535-47bb-0310-9956-ffa450edef68