added ibm lmic config notes and example

Sparkfun-Pro-RF/longfi-us915/README.md

@@ -8,6 +8,13 @@ From the Arduino IDE, open the Library Manager (Sketch->Include Library->Manage
 
 [IBM LMIC framework](https://github.com/matthijskooijman/arduino-lmic)  
 
+### Required Configuration
+This library requires that a config file be setup properly. After you have installed the `IBM LMIC framework` library, navigate to it's directory on your operating system found below. Next, replace the `config.h` file in this directory with this [config.h](config.h) file.
+
+linux: /home/{user}/Arduino/libraries/IBM_LMIC_framework/src/lmic  
+windows: Documents/Arduino/libraries/IBM_LMIC_framework/src/lmic  
+mac os: Documents/Arduino/libraries/IBM_LMIC_framework/src/lmic  
+
 ## Required Arduino Board Support
 
 ### Arduino SAMD Boards (32-bits ARM Cortex-M0+) + SparkFun SAMD Boards