Add installation instructions for Windows/Cygwin64 (#571)

Co-authored-by: Jim Garrison <bitbucket@jhmg.net>

docs/installation.rst

@@ -575,6 +575,54 @@ Docker
 You can also :ref:`Install the Docker <docker-install>` container on Windows. Ensure that
 your command prompt can run the docker "hello world" container.
 
+
+Installing on Cygwin64 under Windows
+====================================
+
+First install the the following prerequisite Cygwin packages using ``setup-x86_64.exe``::
+
+    python36 (or later)
+    python3?-devel
+    python3?-pip
+    python3?-lxml
+    python3?-imaging
+
+       (where 3? means match the version of python3 you installed)
+
+    gcc-g++
+    ghostscript (<=9.50 or >=9.52-2 see note below)
+    libexempi3
+    libexempi-devel
+    libffi6
+    libffi-devel
+    pngquant
+    qpdf
+    libqpdf-devel
+    tesseract-ocr
+    tesseract-ocr-devel
+
+   Note: The Cygwin package for Ghostscript in versions 9.52 and
+   9.52-1 contained a bug that caused an exception to occur when
+   ocrmypdf invoked gs.  Make sure you have either 9.50 (or earlier)
+   or 9.52-2 (or later).
+
+Then open a Cygwin terminal (i.e. ``mintty``), run the following commands.
+Note that if you are using the version of ``pip`` that was installed with the
+Cygwin Python package, the command name will be ``pip3``.  If you have since
+updated ``pip`` (with, for instance ``pip3 install --upgrade pip``) the the
+command is likely just ``pip`` instead of ``pip3``:
+
+.. code-block:: bash
+
+    pip3 install wheel
+    pip3 install ocrmypdf
+
+There is one optional dependency, "unpaper" that is currently not
+available under Cygwin. Without it, certain options such as --clean
+will produce an error message. However, the OCR-to-text-layer
+functionality is available.
+
+
 Installing with Python pip
 ==========================