[README] improve installation section

README.md

@@ -107,10 +107,17 @@ Please note that the following requirements need to be installed:
 Frackit requires the [OpenCascade][2] library to be installed on your system.
 You can download the source code [HERE][2],
 and details on the installation can be found [HERE][10].
-Please note that [OpenCascade][2] requires further 3rd party libraries, of which
+Please note that [OpenCascade][2] requires further 3rd party products, of which
 the mandatory ones are Tcl/Tk and FreeType (see this [link][11]).
-On Linux systems, both of these can be installed from the command line using the
-package manager. For more details, see [HERE][11].
+Note that on Linux systems, the Tcl/Tk libraries can be installed from the command
+line. For instance, if you use Ubuntu, you can install (version 8.6) by writing
+
+```cpp
+sudo apt-get libtk8.6
+sudo apt-get libtcl8.6
+```
+
+in a terminal. For more details on how to install them from the sources, see [HERE][11].
 
 ### Building Frackit under Linux
 After [OpenCascade][2] and the other requirements listed above have been installed,