Update README.md

Code snippet don't compile.
It's missing a step (see https://github.com/mattes/migrate/blob/master/source/go-bindata/go-bindata_test.go)

source/go-bindata/README.md

@@ -24,8 +24,13 @@ func main() {
     func(name string) ([]byte, error) {
       return migrations.Asset(name)
     })
+    
+	d, err := WithInstance(s)
+	if err != nil {
+		t.Fatal(err)
+	}
 
-  m, err := migrate.NewWithSourceInstance("go-bindata", s, "database://foobar")
+  m, err := migrate.NewWithSourceInstance("go-bindata", d, "database://foobar")
   m.Up() // run your migrations and handle the errors above of course
 }
 ```