Add forgotten file describing the silk icon set.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2402 44740490-163a-0410-bde0-09ae8108e29a

www/resources/icons/silk/readme.txt

+These images are from the silk icon set, and are licensed under the
+Creative Commons Attribution 2.5 License.
+
+See:
+http://famfamfam.com/lab/icons/silk/