doc/overview.md: link to CONTRIBUTING.md in the root
doc/overview.md: link to CONTRIBUTING.md in the root

Change-Id: Ifa2275e56f71e1abbfbeebb83c6749f9d106cbd4
Reviewed-on: https://upspin-review.googlesource.com/7923
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin