net: add example for CIDRMask

net: add example for CIDRMask

I had trouble translating the documentation language into a subnet
- e.g. whether /31 was CIDRMask(1, 31) or CIDRMask(1, 32) or
CIDRMask(31, 32) so I thought I'd add a short example showing how to
create the right masks.

Change-Id: Ia6a6de08c5c30b6d2249b3194cced2d3c383e317
Reviewed-on: https://go-review.googlesource.com/32677
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>