design: add 24301-versioned-go

As a reminder, it's fine to make comments about grammar, wording,
and the like on Gerrit, but comments about the substance of the
proposal should be made on GitHub: golang.org/issue/24301.

For golang/go#24301.

Change-Id: I5dcf204da3ebe947eecad7d020002dd52f64faa2
Reviewed-on: https://go-review.googlesource.com/101678
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: rsc