fix out of range problem and write doc

dht22/dht22.go

@@ -104,10 +104,12 @@ func (d *Dev) Read() (physic.Env, error) {
     }
 
     // Verify checksum
+    if len(data) >= 5{
         checksum := data[0] + data[1] + data[2] + data[3]
         if data[4] != checksum {
             return physic.Env{}, errors.New("dht22: checksum error")
         }
+    }
 
     // Parse data (big-endian)
     humidityInt := uint16(data[0])<<8 | uint16(data[1])

dht22/doc.go

@@ -0,0 +1,15 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+//package dht22 is a driver for dht22/am2302 temperature and humidity sensor
+//
+//to start use function: "New"
+//
+//to read data from use function: "Read"
+//or u can read data continiusly from chanale whate create by function: "SenseContinuous"
+//
+//datasheet:
+//https://cdn-shop.adafruit.com/datasheets/Digital+humidity+and+temperature+sensor+AM2302.pdf
+
+package dht22