docs: update installation instructions

* Update pip installation instructions to work
  without setup.py.

* Remove duplication of installation instructions
  in two places.

README.rst

@@ -30,87 +30,9 @@ common case:
 Obtaining sshuttle
 ------------------
 
-- Ubuntu 16.04 or later::
+Please see the documentation_.
 
-      apt-get install sshuttle
+.. _Documentation: https://sshuttle.readthedocs.io/en/stable/installation.html
-
-- Debian stretch or later::
-
-      apt-get install sshuttle
-
-- Arch Linux::
-
-      pacman -S sshuttle
-
-- Fedora::
-
-      dnf install sshuttle
-
-- openSUSE::
-
-      zypper in sshuttle
-
-- Gentoo::
-
-      emerge -av net-proxy/sshuttle
-
-- NixOS::
-
-      nix-env -iA nixos.sshuttle
-
-- From PyPI::
-
-      sudo pip install sshuttle
-
-- Clone::
-
-      git clone https://github.com/sshuttle/sshuttle.git
-      cd sshuttle
-      sudo ./setup.py install
-
-- FreeBSD::
-
-      # ports
-      cd /usr/ports/net/py-sshuttle && make install clean
-      # pkg
-      pkg install py39-sshuttle
-
-- OpenBSD::
-
-      pkg_add sshuttle
-
-- macOS, via MacPorts::
-
-      sudo port selfupdate
-      sudo port install sshuttle
-
-It is also possible to install into a virtualenv as a non-root user.
-
-- From PyPI::
-
-      virtualenv -p python3 /tmp/sshuttle
-      . /tmp/sshuttle/bin/activate
-      pip install sshuttle
-
-- Clone::
-
-      virtualenv -p python3 /tmp/sshuttle
-      . /tmp/sshuttle/bin/activate
-      git clone https://github.com/sshuttle/sshuttle.git
-      cd sshuttle
-      ./setup.py install
-
-- Homebrew::
-
-      brew install sshuttle
-
-- Nix::
-
-      nix-shell -p sshuttle
-
-- Windows::
-
-      pip install sshuttle
 
 Documentation
 -------------

docs/installation.rst

@@ -1,23 +1,84 @@
 Installation
 ============
 
+- Ubuntu 16.04 or later::
+
+      apt-get install sshuttle
+
+- Debian stretch or later::
+
+      apt-get install sshuttle
+
+- Arch Linux::
+
+      pacman -S sshuttle
+
+- Fedora::
+
+      dnf install sshuttle
+
+- openSUSE::
+
+      zypper in sshuttle
+
+- Gentoo::
+
+      emerge -av net-proxy/sshuttle
+
+- NixOS::
+
+      nix-env -iA nixos.sshuttle
+
 - From PyPI::
 
-      pip install sshuttle
+      sudo pip install sshuttle
-
-- Debian package manager::
-
-      sudo apt install sshuttle
 
 - Clone::
 
       git clone https://github.com/sshuttle/sshuttle.git
       cd sshuttle
-      ./setup.py install
+      sudo ./setup.py install
 
+- FreeBSD::
 
-Optionally after installation
+      # ports
------------------------------
+      cd /usr/ports/net/py-sshuttle && make install clean
+      # pkg
+      pkg install py39-sshuttle
 
-- Install sudoers configuration. For details, see the "Sudoers File" section in :doc:`usage`
+- OpenBSD::
 
+      pkg_add sshuttle
+
+- macOS, via MacPorts::
+
+      sudo port selfupdate
+      sudo port install sshuttle
+
+It is also possible to install into a virtualenv as a non-root user.
+
+- From PyPI::
+
+      python3 -m venv /tmp/sshuttle
+      . /tmp/sshuttle/bin/activate
+      pip install sshuttle
+
+- Clone::
+
+      git clone https://github.com/sshuttle/sshuttle.git
+      cd sshuttle
+      python3 -m venv /tmp/sshuttle
+      . /tmp/sshuttle/bin/activate
+      python -m pip install .
+
+- Homebrew::
+
+      brew install sshuttle
+
+- Nix::
+
+      nix-shell -p sshuttle
+
+- Windows::
+
+      pip install sshuttle