docs/README: Add note to make OS requirement clear

Author: shymega-ct

README.rst

@@ -110,6 +110,11 @@ To get started, first `install BuildStream by following the installation guide
 and then follow our tutorial in the
 `user guide <https://docs.buildstream.build/master/main_using.html>`_.
 
+.. TIP::
+  *Buildstream works on POSIX-compliant (or other UNIX-like) systems**, though
+  you may be limited by sandbox implementations in Buildbox.
+  Linux.
+
 We also recommend exploring some existing BuildStream projects:
 
 * https://gitlab.gnome.org/GNOME/gnome-build-meta/