Add import comments.

The repository has been recently renamed from "examples" to "example"
in #58. Help make the new expected import path more clear by adding
import comments. Reference: https://golang.org/cmd/go/#hdr-Import_path_checking.

This way, the expected import path is visible in the source code, in
addition to README. It also gives a better error message when trying
to go get or go install the package with incorrect old import path.

Closes #58 (again).

gl21-cube/cube.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Renders a textured spinning cube using GLFW 3 and OpenGL 2.1.
-package main
+package main // import "github.com/go-gl/example/gl21-cube"
 
 import (
 	"go/build"

gl41core-cube/cube.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Renders a textured spinning cube using GLFW 3 and OpenGL 4.1 core forward-compatible profile.
-package main
+package main // import "github.com/go-gl/example/gl41core-cube"
 
 import (
 	"fmt"