updated Ubuntu packages required

README.rst

@@ -121,14 +121,15 @@ Before building on Linux systems you must install the following packages:
 .. code-block :: bash
 
   sudo apt-get install build-essential libgl1-mesa-dev libxt-dev
+  sudo apt-get libx11-xcb-dev libxcb1-dev libxcb-glx0-dev libxkbcommon-x11-dev
   sudo apt-get install libexpat1-dev libgtk2.0-dev liblapack-dev
 
 On Linux, to optionally build with Python and to build the user documentation,
 you will also need to install the following:
 
 .. code-block :: bash
 
-  sudo apt-get install python3-dev python3-sphinx-rtd-theme
+  sudo apt-get install python3-dev python3-sphinx python3-sphinx-rtd-theme
 
 Set up the folder structure and obtain the source files. This can be done with
 git or by downloading the files and extracting them. Then setup the folder(s)