ExampleSliceStable echoes the sort.Slice example, to demonstrate sorting
on two fields together preserving order between sorts.

Change-Id: I8afc20c0203991bfd57260431eda73913c165355
Reviewed-on: https://go-review.googlesource.com/37196Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>