Add Sage.

EXAMPLES

@@ -53,6 +53,7 @@ included, please mail to `the Google group
 * Quex: http://quex.sourceforge.net/
 * Reteisi: http://docs.argolinux.org/reteisi/
 * Roundup: http://www.roundup-tracker.org/
+* Sage: http://sagemath.org/doc/
 * Satchmo: http://www.satchmoproject.com/docs/svn/
 * Scapy: http://www.secdev.org/projects/scapy/doc/
 * Self: http://selflanguage.org/