adding note about go version 1.5 requirement for hardware signing

Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

(cherry picked from commit 1cc950b52ecd990016275f7df3bb21685284c79a)

From PR #20112

project/PACKAGERS.md

@@ -44,7 +44,8 @@ need to package Docker your way, without denaturing it in the process.
 To build Docker, you will need the following:
 
 * A recent version of Git and Mercurial
-* Go version 1.4 or later
+* Go version 1.4 or later (Go version 1.5 or later required for hardware signing
+  support in Docker Content Trust)
 * A clean checkout of the source added to a valid [Go
   workspace](https://golang.org/doc/code.html#Workspaces) under the path
   *src/github.com/docker/docker* (unless you plan to use `AUTO_GOPATH`,