works on ChromeOS with Crostini VM (#262)

* works on ChromeOS with Crostini VM

tested on ASUS C101PA on Dev channel, should also work on Intel machines and Beta channel

* crostini doc, and a note about xterms and VNC

tested on ASUS C101PA on Dev channel, should also work on Intel machines and Beta channel

docs/chromeos.rst

+Google ChromeOS
+===============
+
+Currently there is no built in support for running sshuttle directly on
+Google ChromeOS/Chromebooks.
+
+What we can really do is to create a Linux VM with Crostini.  In the default
+stretch/Debian 9 VM, you can then install sshuttle as on any Linux box and
+it just works, as do xterms and ssvncviewer etc.
+
+https://www.reddit.com/r/Crostini/wiki/getstarted/crostini-setup-guide
+

docs/platform.rst

@@ -6,5 +6,6 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   chromeos
    tproxy
    windows