docs: Add link to upgrade notes for version 1.7.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2690 44740490-163a-0410-bde0-09ae8108e29a

docs/index.txt

@@ -2,6 +2,7 @@ SimpleSAMLphp Documentation
 ===========================
 
  * [Installing simpleSAMLphp](simplesamlphp-install)
+    * [Upgrade notes for version 1.7](simplesamlphp-upgrade-notes-1.7)
     * [Upgrade notes for version 1.6](simplesamlphp-upgrade-notes-1.6)
     * [Upgrade notes for version 1.5](simplesamlphp-upgrade-notes-1.5)
     * [Installation from Subversion](simplesamlphp-subversion)