document 'since' pagination for Users.ListAll

Fixes #278.

Change-Id: Ia1d0c1f6b395d493714d91e0f6e79dc2506f8fde

github/users.go

@@ -144,6 +144,22 @@ type UserListOptions struct {
 
 // ListAll lists all GitHub users.
 //
+// To paginate through all users, populate 'Since' with the ID of the last user.
+// Example (warning: this will quickly eat through your GitHub API quota!):
+//
+// 	opts := &github.UserListOptions{}
+// 	for {
+// 		users, _, err := client.Users.ListAll(opts)
+// 		if err != nil {
+// 			log.Fatalf("error listing users: %v", err)
+// 		}
+// 		if len(users) == 0 {
+// 			break
+// 		}
+// 		opts.Since = *users[len(users)-1].ID
+// 		// Process users...
+// 	}
+//
 // GitHub API docs: http://developer.github.com/v3/users/#get-all-users
 func (s *UsersService) ListAll(opt *UserListOptions) ([]User, *Response, error) {
 	u, err := addOptions("users", opt)