Document that the library also supports MKR WAN 1310

[per1234]

Previously, text gave the impression that the library only supported the MKR WAN 1300.

This adresses #70, but does not completely fix that issue because the documentation on arduino.cc also needs to be updated:

https://www.arduino.cc/en/Reference/Libraries

MKR WAN 1300 only Library

• MKRWAN - library to be used only with MKR WAN 1300

• https://www.arduino.cc/en/Reference/MKRWAN
• https://www.arduino.cc/en/Tutorial/MKRWANLoraSendAndReceive
• https://www.arduino.cc/en/Tutorial/MKRWANFirstConfiguration
• https://www.arduino.cc/en/Reference/MKRWANBegin
• https://www.arduino.cc/en/Reference/MKRWANRestart
• https://www.arduino.cc/en/Reference/MKRWANVersion
• https://www.arduino.cc/en/Reference/MKRWANDeviceEUI
• https://www.arduino.cc/en/Reference/MKRWANJoinOTAA
• https://www.arduino.cc/en/Reference/MKRWANJoinABP
• https://www.arduino.cc/en/Reference/MKRWANBeginPacket
• https://www.arduino.cc/en/Reference/MKRWANRead
• https://www.arduino.cc/en/Reference/MKRWANPrint
• https://www.arduino.cc/en/Reference/MKRWANAvailable
• https://www.arduino.cc/en/Reference/MKRWANEndPacket