Add docs for installation on Windows

docs/base/install/common.rst

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+
+.. highlight:: sh
+
+Common Install Steps
+====================
+
+How exactly you install an app to your virtual environment can vary.
+
+However they all involve using ``pip install`` in some way or another.  And
+therefore your virtual environment should be "activated" when running the
+commands shown below.
+
+
+Installing from PyPI
+--------------------
+
+If the app is published to the `Python Package Index`_ (PyPI) then you can just
+``pip install`` it.  For instance that is the case for `Theo`_, so if that's
+what you're after then::
+
+   pip install tailbone-theo
+
+.. _Python Package Index: https://pypi.org/
+.. _Theo: https://pypi.org/project/tailbone-theo/
+
+
+Installing from Source
+----------------------
+
+If you need (or want) to install from source, then of course the first step is
+to obtain the source code for the app.  Here we'll again use Theo as our example::
+
+   git clone https://kallithea.rattailproject.org/rattail-project/theo
+
+Then you would install that to your virtual environment like so::
+
+   pip install -e ./theo
+
+
+Sanity Check
+------------
+
+Before we do anything else let's confirm that you have the 'rattail' package
+installed, at the very least.  With your env still activated try this::
+
+   rattail --version
+
+That of course should spit out a version number; if you see anything else then
+you may need to troubleshoot that first.