Document that Lab only supports Linux.

INSTALL.txt

@@ -1,12 +1,12 @@
 Install Lab
 -----------
 
-Lab supports Python 3.6+. We recommend installing Lab in a `Python
-virtual environment <https://docs.python.org/3/tutorial/venv.html>`_.
-This has the advantage that there are no modifications to the
-system-wide configuration, and that you can create multiple environments
-with different Lab versions (e.g., for different papers) without
-conflicts::
+Lab requires Python 3.6+ and Linux (e.g., Ubuntu). We recommend installing
+Lab in a `Python virtual environment
+<https://docs.python.org/3/tutorial/venv.html>`_. This has the advantage
+that there are no modifications to the system-wide configuration, and that
+you can create multiple environments with different Lab versions (e.g.,
+for different papers) without conflicts::
 
     # Install required packages, including virtualenv.
     sudo apt install python3 python3-venv