design/55022-pgo.md: add design doc

We propose adding support for profile-guided optimization (PGO) to
Go.

For golang/go#55022.

Change-Id: I9bf8afc4858a530bc32fc2b40698209a45695203
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/430355
Reviewed-by: Michael Pratt <[email protected]>

Author: cherrymui