Add small note on easy download script

I'm not sure the existence/option of downloading all the datasets via bash script will be obvious to people approaching the repo via the written tutorial.

Author: kirkins

doc/gettingstarted.txt

@@ -22,6 +22,11 @@ On each learning algorithm page, you will be able to download the corresponding
 
     git clone https://github.com/lisa-lab/DeepLearningTutorials.git
 
+On linux systems, after cloning, all datasets can be downloaded at once with:
+
+    cd DeeepLearningTutorials/data
+    ./download.sh
+    
 
 .. _datasets: