Added how to install tkinter for plotting in Python3.

doc/erko_howto_tools.txt

@@ -701,6 +701,8 @@ Start --> Administration --> Synaptic package manager
 > sudo pip install numpy      # to run Python2 library installer as root
 > sudo pip3 install numpy     # to run Python3 library installer as root
 
+> sudo apt-get install python3-tk # worked, now I can do: python3 test_plot.py
+
 > sudo apt-get install pip    # to install Python2 library installer
 > sudo apt-get install python-matplotlib