Update CHANGELOG

CHANGELOG.md

 ## 0.5.2 (unreleased)
 
+IMPROVEMENTS:
+
+* core: Most downloads made by Packer now use a custom user agent. [GH-803]
+
 BUG FIXES:
 
 * core: Fix crash case if blank parameters are given to Packer. [GH-832]

packer/version.go

@@ -39,6 +39,9 @@ func (versionCommand) Synopsis() string {
 	return "print Packer version"
 }
 
+// VersionString returns the Packer version in human-readable
+// form complete with pre-release and git commit info if it is
+// available.
 func VersionString() string {
 	var versionString bytes.Buffer
 	fmt.Fprintf(&versionString, "Packer v%s", Version)