design: add 18802-percpu-sharded.md
design: add 18802-percpu-sharded.md

For golang/go#18802.

Change-Id: I10678de235500dc9a9a217e4cf114731c8bd1162
GitHub-Last-Rev: 4cec2dbfd52a4fe71747399a693e4a26b946f98f
GitHub-Pull-Request: golang/proposal#13
Reviewed-on: https://go-review.googlesource.com/118135
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal