design/36606-64-bit-field-alignment: add proposal

This change adds a new design proposal to the repository describing
changes to Go alignment behavior and addition of Go directives, as a way
to deal with the problems associated with issue golang/go#599.

Updates golang/go#36606

Change-Id: I2b09424a71d2ffed7136205756f6e101aafd13bb
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/237317
Reviewed-by: Austin Clements <[email protected]>

Author: danscales