add SoftwareSerial library limitation

README.md

@@ -13,6 +13,16 @@ Only two special functions are different for SIM900 Shield or SIM800L PCB boards
 
 GPRS Shield provide you a way to DIY your own **mobile phone**. You can use your GRPS Shield to dial a phone number or send a text message to your friend. Besides these, you can use it to access the network and view web pages. For more information, you can visit our [Wiki Page](http://wiki.seeedstudio.com/GPRS_Shield_V3.0/)
 
+[Limitations of SoftwareSerial library](https://www.arduino.cc/en/Reference/SoftwareSerial)
+===
+
+If using multiple software serial ports, only one can receive data at a time.
+Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15, 50, 51, 52, 53, A8 (62), A9 (63), A10 (64), A11 (65), A12 (66), A13 (67), A14 (68), A15 (69).
+Not all pins on the Leonardo and Micro support change interrupts, so only the following can be used for RX: 8, 9, 10, 11, 14 (MISO), 15 (SCK), 16 (MOSI).
+On Arduino or Genuino 101 the current maximum RX speed is 57600bps
+On Arduino or Genuino 101 RX doesn't work on Pin 13
+If your project requires simultaneous data flows, see Paul Stoffregen's AltSoftSerial library. AltSoftSerial overcomes a number of other issues with the core SoftwareSerial, but has it's own limitations. Refer to the AltSoftSerial site for more information.
+
 Features
 ===
 + Compatible with standard Arduino and Arduino Mega