Update PACKAGERS.md to explicitly talk about stripping, quoting Dave Cheney from Golang upstream (who is very, very adamant about not stripping Golang compiled binaries)

While there, remove the now-outdated section about `CGO_ENABLED=0`

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3899 from tianon/do-not-strip-binaries

Update PACKAGERS.md to explicitly talk about stripping