io/i2c: add an example for Open

Change-Id: Ie9ad7a09b91f53372107dae19cb01dfd09978a94
Reviewed-on: https://go-review.googlesource.com/22517
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Johan Euphrosine <proppy@google.com>
diff --git a/io/i2c/example_test.go b/io/i2c/example_test.go
new file mode 100644
index 0000000..535608c
--- /dev/null
+++ b/io/i2c/example_test.go
@@ -0,0 +1,22 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package i2c_test
+
+import (
+	"golang.org/x/exp/io/i2c"
+)
+
+func ExampleOpen() {
+	d, err := i2c.Open(&i2c.Devfs{}, 1, 0x39)
+	if err != nil {
+		panic(err)
+	}
+
+	// opens a 10-bit address
+	d, err = i2c.Open(&i2c.Devfs{}, 1, i2c.TenBit(0x78))
+	if err != nil {
+		panic(err)
+	}
+}