[tools] Mention that need to install `psutil` Python module

[dimakuv]

Commit [tools] Introduce Gramine VM ID in gramine-vm.in introduces a requirement of psutil Python module. Without it installed on the host, gramine-vm fails like this:

$ gramine-vm helloworld
Error: Python script for determining the Gramine-VM ID exited with a non-zero status: 1
Error message: Traceback (most recent call last):
  File "<string>", line 2, in <module>
ModuleNotFoundError: No module named 'psutil'

To fix the problem, simply apt install python3-psutil.

We must mention it in the documentation.