docs: Add Vagrant instructions for Debian "stretch".

Aka the current "testing" release, expected to graduate to "stable"
later in 2017.

Fortunately the instructions are very similar to those for
Ubuntu 16.04 and 14.04 -- two packages don't exist, and
those two packages turn out (empirically, on my laptop)
not to be necessary.

Leave most references to "Ubuntu" still just saying "Ubuntu",
on the theory that Debian users will generally follow those
breadcrumbs where they lead and in order to keep lists short.

docs/dev-env-first-time-contributors.md

@@ -48,6 +48,7 @@ proxy](#specifying-a-proxy) if you need a proxy to access the internet.)
 - **macOS**: macOS (10.11 El Capitan or 10.12 Sierra recommended), Git,
   [VirtualBox][vbox-dl], [Vagrant][vagrant-dl-macos].
 - **Ubuntu**: 14.04 64-bit or 16.04 64-bit, Git, [Vagrant][vagrant-dl-deb], lxc.
+  - or **Debian**: 9.0 "stretch" 64-bit
 - **Windows**: Windows 64-bit (Win 10 recommended), hardware
   virtualization enabled (VT-X or AMD-V), administrator access,
   [Git for Windows][git-bash] (which installs Git BASH), [VirtualBox][vbox-dl],
@@ -62,6 +63,7 @@ Jump to:
 
 * [macOS](#macos)
 * [Ubuntu](#ubuntu)
+* [Debian](#debian)
 * [Windows](#windows-10)
 
 #### macOS
@@ -152,6 +154,32 @@ this](#permissions-errors).
 
 Now you are ready for [Step 2: Get Zulip Code.](#step-2-get-zulip-code)
 
+#### Debian
+
+The setup for Debian 9.0 "stretch" is just like [for Ubuntu 16.04](#ubuntu),
+with one difference.
+
+If you're in a hurry, you can copy and paste the following into your terminal
+after which you can jump to [Step 2: Get Zulip Code](#step-2-get-zulip-code):
+
+```
+sudo apt-get -y purge vagrant && \
+wget https://releases.hashicorp.com/vagrant/1.8.6/vagrant_1.8.6_x86_64.deb && \
+sudo dpkg -i vagrant*.deb && \
+sudo apt-get -y install build-essential git ruby lxc redir && \
+vagrant plugin install vagrant-lxc && \
+vagrant lxc sudoers
+```
+
+For a step-by-step explanation, follow the [Ubuntu instructions above](#ubuntu),
+with the following difference: in "2. Install remaining dependencies", the
+command is
+
+```
+sudo apt-get install build-essential git ruby lxc redir
+```
+
+
 #### Windows 10
 
 1. Install [Git for Windows][git-bash], which installs *Git BASH*.