give guidance to reduce scope of constants

src/var-scope.md

@@ -1,6 +1,6 @@
-# Reduce Scope of Variables
+# Reduce Scope of Variables and Constants
 
-Where possible, reduce scope of variables. Do not reduce the scope if it
+Where possible, reduce scope of variables and constants. Do not reduce the scope if it
 conflicts with [Reduce Nesting](nest-less.md).
 
 <table>
@@ -66,3 +66,36 @@ return nil
 
 </td></tr>
 </tbody></table>
+
+Avoid making constants global unless they are used in multiple functions.
+
+<table>
+<thead><tr><th>Bad</th><th>Good</th></tr></thead>
+<tbody>
+<tr><td>
+
+```go
+const (
+  _defaultPort = 8080
+  _defaultUser = "user"
+)
+
+func Bar() {
+  fmt.Println("Default port", _defaultPort)
+}
+```
+
+</td><td>
+
+```go
+func Bar() {
+  const (
+    defaultPort = 8080
+    defaultUser = "user"
+  )
+  fmt.Println("Default port", _defaultPort)
+}
+```
+
+</td></tr>
+</tbody></table>