Error checking added in the example. Functions description added

lua_modules/dht22/README.md

@@ -1,5 +1,8 @@
 # DHT22 module
 
+This module is compatible with DHT22 and DHT21.  
+No need to use a resistor to connect the pin data of DHT22 to ESP8266.
+
 ## Example  
 ```lua
 PIN = 4 --  data pin, GPIO2
@@ -9,14 +12,41 @@ dht22.read(PIN)
 t = dht22.getTemperature()
 h = dht22.getHumidity()
 
+if h == -1 then
+  print("Error reading from DHT22")
+else
   -- temperature in degrees Celsius  and Farenheit
   print("Temperature: "..(t / 10).."."..(t % 10).." deg C")
   print("Temperature: "..(9 * t / 50 + 32).."."..(9 * t / 5 % 10).." deg F")
 
   -- humidity
   print("Humidity: "..(h/10).."."..(h%10).."%")
+end
 
 -- release module
 dht22 = nil
 package.loaded["dht22"]=nil
 ```
+## Functions
+### read
+read(pin) 
+Read humidity and temperature from DHT22.
+
+**Parameters:**
+
+* pin - ESP8266 pin connect to data pin in DHT22
+
+### getHumidity
+getHumidity()  
+Returns the humidity of the last reading.
+
+**Returns:**  
+* last humidity reading in per thousand
+
+### getTemperature
+getTemperature()  
+Returns the temperature of the last reading.
+
+**Returns:**  
+* last temperature reading in 0.1ºC
+