Added ExifTool to install instructions

Author: anderskm

README.md

@@ -13,7 +13,9 @@ Drone Dataflow - A MATLAB toolbox for processing images captured by a UAV
 
 ## Installation
 1. Download and unzip the toolbox: https://github.com/anderskm/drone-dataflow/archive/master.zip
-2. Start MATLAB in the folder, where you unzipped the toolbox.
-3. Write "dronedataflow" in the command window to open the Drone Dataflow toolbox GUI.
-4. Select the desired task and follow the instructions.
+2. Download and install ExifTool (https://www.sno.phy.queensu.ca/~phil/exiftool/)
+3. Start MATLAB in the folder, where you unzipped the toolbox.
+4. Write "dronedataflow" in the command window to open the Drone Dataflow toolbox GUI.
+5. Select the desired task and follow the instructions.
     * **Note**: Check MATLABs command window for progress and details.
+    * **Note**: The first time ExifTool is called, you will be prompted to locate it on your pc.