Add cacheserver writethrough config example
Add cacheserver writethrough config example

Change-Id: Id151a07dbb0db321e0407b2eb79d307cccd6f5f3
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19560
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin