Describe multicast APIs in readme, update keywords.

README.md

@@ -75,6 +75,14 @@ This is mostly similar to WiFi shield library. Differences include:
 - ```WiFi.localIP()``` is for STA, ```WiFi.softAPIP()``` is for AP.
 - ```WiFi.RSSI()``` doesn't work
 - ```WiFi.printDiag(Serial);``` will print out some diagnostic info
+- ```WiFiUDP``` class supports sending and receiving multicast packets on STA interface.
+When sending a multicast packet, replace ```udp.beginPacket(addr, port)``` with 
+```udp.beginPacketMulticast(addr, port, WiFi.localIP())```.
+When listening to multicast packets, replace ```udp.begin(port)``` with 
+```udp.beginMulticast(WiFi.localIP(), multicast_ip_addr, port)```.
+You can use ```udp.destinationIP()``` to tell whether the packet received was
+sent to the multicast or unicast address.
+Also note that multicast doesn't work on softAP interface.
 
 WiFiServer, WiFiClient, and WiFiUDP behave mostly the same way as with WiFi shield library.
 Four samples are provided for this library.

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/keywords.txt

@@ -53,6 +53,7 @@ remotePort	KEYWORD2
 softAP	KEYWORD2
 softAPIP	KEYWORD2
 softAPmacAddress	KEYWORD2
+softAPConfig	KEYWORD2
 printDiag	KEYWORD2
 hostByName	KEYWORD2
 scanNetworks	KEYWORD2