Improve installation documentation

Resolves pypa#1618.
Resolves pypa#1619.
Resolves pypa#1620.

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

docs/changelog/1618.doc.rst

@@ -0,0 +1,2 @@
+Document requirements (pip + index server) when installing via pip under the installation section - by
+:user:`gaborbernat`.

docs/changelog/1619.doc.rst

@@ -0,0 +1 @@
+Document installing from non PEP-518 systems - :user:`gaborbernat`.

docs/changelog/1620.doc.rst

@@ -0,0 +1 @@
+Document installing latest unreleased version from Github - :user:`gaborbernat`.

docs/installation.rst

@@ -19,13 +19,31 @@ via pip
 Alternatively you can install it within the global Python interpreter itself (perhaps as a user package via the
 ``--user`` flag). Be cautious if you are using a python install that is managed by your operating system or
 another package manager. ``pip`` might not coordinate with those tools, and may leave your system in an
-inconsistent state.
+inconsistent state. Note, if you go down this path you need to ensure pip is new enough per the subsections below:
 
 .. code-block:: console
 
     python -m pip install --user virtualenv
     python -m virtualenv --help
 
+wheel
+~~~~~
+Installing virtualenv via a wheel (default with pip) requires an installer that can understand the ``python-requires``
+tag (see `PEP-503 <https://www.python.org/dev/peps/pep-0503/>`_), with pip this is version ``9.0.0`` (released 2016
+November). Furthermore, in case you're not installing it via the PyPi you need to be using a mirror that correctly
+forwards the ``python-requires`` tag (notably the OpenStack mirrors don't do this, or older
+`devpi <https://github.com/devpi/devpi>`_ versions - added with version ``4.7.0``).
+
+.. _sdist:
+
+sdist
+~~~~~
+When installing via a source distribution you need an installer that handles the
+`PEP-517 <https://www.python.org/dev/peps/pep-0517/>`_ specification. In case of ``pip`` this is version ``18.0.0`` or
+later (released on 2018 July). If you cannot upgrade your pip to support this you need to ensure that the build
+requirements from `pyproject.toml <https://github.com/pypa/virtualenv/blob/master/pyproject.toml#L2>`_ are satisfied
+before triggering the install.
+
 via zipapp
 ----------
 
@@ -43,6 +61,23 @@ minor release use the link ``https://bootstrap.pypa.io/virtualenv/x.y/virtualenv
 supporting Python 2.7 use
 `https://bootstrap.pypa.io/virtualenv/2.7/virtualenv.pyz <https://bootstrap.pypa.io/2.7/virtualenv/virtualenv.pyz>`_.
 
+via ``setup.py``
+----------------
+We don't recommend and officially support this method. One should prefer using an installer that supports
+`PEP-517 <https://www.python.org/dev/peps/pep-0517/>`_ interface, such as pip ``19.0.0`` or later. That being said you
+might be able to still install a package via this method if you satisfy build dependencies before calling the install
+command (as described under :ref:`sdist`).
+
+latest unreleased
+-----------------
+Installing an unreleased version is discouraged and should be only done for testing purposes. If you do so you'll need
+a pip version of at least ``18.0.0`` and use the following command:
+
+
+.. code-block:: console
+
+    pip install git+https://github.com/pypa/virtualenv.git@master
+
 .. _compatibility-requirements:
 
 Python and OS Compatibility