Update README.md

Add note for #34.

Author: sandeepmistry

README.md

@@ -27,6 +27,8 @@ An [Arduino](http://arduino.cc/) library for sending and receiving data using [L
 
 `NSS`, `NRESET`, and `DIO0` pins can be changed by using `LoRa.setPins(ss, reset, dio0)`. `DIO0` pin is optional, it is only needed for receive callback mode. If `DIO0` pin is used, it **must** be interrupt capable via [`attachInterrupt(...)`](https://www.arduino.cc/en/Reference/AttachInterrupt).
 
+**NOTE**: Some boards (like the Arduino Nano), cannot supply enough current for the SX127x in TX mode. This will cause lockups when sending, be sure to use an external 3.3V supply that can provide at least 120mA's when using these boards.
+
 ## Installation
 
 ### Using the Arduino IDE Library Manager