content: link to and describe the fibonacci sequence

Fixes golang/go#14044

Change-Id: Icc39515fcb8a6c9601b18f1c3a77612e3a8072e0
Reviewed-on: https://go-review.googlesource.com/18780
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Francesc Campoy Flores <campoy@golang.org>
1 file changed