[README.rst] Add "Optional Requirements" section

README.rst

@@ -155,6 +155,101 @@ A selection of sample python programs is in the
 `Snippets <https://github.com/fonttools/fonttools/blob/master/Snippets/>`__
 directory.
 
+Optional Requirements
+---------------------
+
+The ``fontTools`` package currently has no (required) external dependencies
+besides the modules included in the Python Standard Library.
+However, a few extra dependencies are required by some of its modules, which
+are needed to unlock optional features.
+
+-  ``Lib/fontTools/ttLib/woff2.py``
+
+   Module to compress/decompress WOFF 2.0 web fonts; it requires:
+
+   -  `brotli <https://pypi.python.org/pypi/Brotli>`__: Python bindings of
+      the Brotli compression library.
+
+-  ``Lib/fontTools/ttLib/sfnt.py``
+
+   To better compress WOFF 1.0 web fonts, the following module can be used
+   instead of the built-in ``zlib`` library:
+
+   -  `zopfli <https://pypi.python.org/pypi/zopfli>`__: Python bindings of
+      the Zopfli compression library.
+
+-  ``Lib/fontTools/unicode.py``
+
+   To display the Unicode character names when dumping the ``cmap`` table
+   with ``ttx`` we use the ``unicodedata`` module in the Standard Library.
+   The version included in there varies between different Python versions.
+   To use the latest available data, you can install:
+
+   -  `unicodedata2 <https://pypi.python.org/pypi/unicodedata2>`__:
+      ``unicodedata`` backport for Python 2.7 and 3.5 updated to the latest
+      Unicode version 9.0. Note this is not necessary if you use Python 3.6
+      as the latter already comes with an up-to-date ``unicodedata``.
+
+-  ``Lib/fontTools/varLib/interpolatable.py``
+
+   Module for finding wrong contour/component order between different masters.
+   It requires one of the following packages in order to solve the so-called
+   "minimum weight perfect matching problem in bipartite graphs", or
+   the Assignment problem:
+
+   *  `scipy <https://pypi.python.org/pypi/scipy>`__: the Scientific Library
+      for Python, which internally uses `NumPy <https://pypi.python.org/pypi/numpy>`__
+      arrays and hence is very fast;
+   *  `munkres <https://pypi.python.org/pypi/munkres>`__: a pure-Python
+      module that implements the Hungarian or Kuhn-Munkres algorithm.
+
+-  ``Lib/fontTools/misc/symfont.py``
+
+   Advanced module for symbolic font statistics analysis; it requires:
+
+   *  `sympy <https://pypi.python.org/pypi/sympy>`__: the Python library for
+      symbolic mathematics.
+
+-  ``fontTools.t1Lib``
+
+   To get the file creator and type of Macintosh PostScript Type 1 fonts
+   on Python 3 you need to install the following module, as the old ``MacOS``
+   module is no longer included in Mac Python:
+
+   *  `xattr <https://pypi.python.org/pypi/xattr>`__: Python wrapper for
+      extended filesystem attributes (macOS platform only).
+
+-  ``Lib/fontTools/pens/cocoaPen.py``
+
+   Pen for drawing glyphs with Cocoa ``NSBezierPath``, requires:
+
+   *  `PyObjC <https://pypi.python.org/pypi/pyobjc>`__: the bridge between
+      Python and the Objective-C runtime (macOS platform only).
+
+-  ``Lib/fontTools/pens/qtPen.py``
+
+   Pen for drawing glyphs with Qt's ``QPainterPath``, requires:
+
+   *  `PyQt5 <https://pypi.python.org/pypi/PyQt5>`__: Python bindings for
+      the Qt cross platform UI and application toolkit.
+
+-  ``Lib/fontTools/pens/reportLabPen.py``
+
+   Pen to drawing glyphs as PNG images, requires:
+
+   *  `reportlab <https://pypi.python.org/pypi/reportlab>`__: Python toolkit
+      for generating PDFs and graphics.
+
+-  ``fontTools.inspect``
+
+   A GUI font inspector, requires one of the following packages:
+
+   *  `PyGTK <https://pypi.python.org/pypi/PyGTK>`__: Python bindings for
+      GTK  2.x (only works with Python 2).
+   *  `PyGObject <https://wiki.gnome.org/action/show/Projects/PyGObject>`__ :
+      Python bindings for GTK 3.x and gobject-introspection libraries (also
+      compatible with Python 3).
+
 Testing
 ~~~~~~~