updates documentation

documentation/sphinx_static/overview.rst

@@ -164,6 +164,8 @@ from random initial conditions, and have a look at the results.
 
 .. code:: bash
 
+    # change directory to a clean "scratch" folder of your choice:
+    cd $SCRATCH
     # depending on how curious you are, you may have a look at the
     # options first:
     turtle --help
@@ -172,7 +174,15 @@ from random initial conditions, and have a look at the results.
     # or you may just run it:
     turtle DNS NSVE -n 32
 
-This launches a simulation in the current folder, with the simulation
+**Note**:
+
+    * Python environment where the TurTLE package was installed must
+      be activated prior to calling the commands.
+
+    * Please use an empty "SCRATCH" folder; in particular running turtle
+      from within the build folder may lead to problems.
+
+This launches a simulation with the simulation
 name "test".
 The simulation itself should not take more than a few seconds, since
 this is just a :math:`32^3` simulation run for 8 iterations.