project,vendor.mod: document more clearly that we are not a Go module

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

project/PACKAGERS.md

@@ -47,6 +47,10 @@ If you would rather (or must, due to distro policy) package these dependencies
 yourself, take a look at "vendor.mod" for an easy-to-parse list of the
 exact version for each.
 
+Note that this project is **not** a Go module. To properly build the project,
+`GOPATH` mode must be used. The project is migrating to Go modules, but `vendor.mod`
+should not be interchanged with `go.mod` until this transition is complete.
+
 ## Stripping Binaries
 
 Please, please, please do not strip any compiled binaries. This is really

vendor.mod

@@ -1,6 +1,7 @@
 // 'vendor.mod' enables use of 'go mod vendor' to managed 'vendor/' directory.
-// There is no 'go.mod' file, as that would imply opting in for all the rules
-// around SemVer, which this repo cannot abide by as it uses CalVer.
+// There is no 'go.mod' file, as the project has not yet completed the migration
+// to a Go module. This project should be built in GOPATH mode, and not module mode,
+// until the migration is complete and this file becomes 'go.mod.'
 
 module github.com/docker/docker