Add README.rst

Author: taldcroft

README.rst

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+Deproject
+====================
+
+``Deproject`` is a CIAO Sherpa extension package to facilitate
+deprojection of two-dimensional annular X-ray spectra to recover the
+three-dimensional source properties.  For typical thermal models this would
+include the radial temperature and density profiles. This basic method 
+has been used extensively for X-ray cluster analysis and is the basis for the
+XSPEC model ``projct``.  The ``deproject`` module brings this
+functionality to *Sherpa* as a Python module that is straightforward to use and
+understand.
+
+The basic physical assumption of ``deproject`` is that the extended source
+emissivity is constant and optically thin within spherical shells whose radii
+correspond to the annuli used to extract the specta.  Given this assumption one
+constructs a model for each annular spectrum that is a linear volume-weighted
+combination of shell models.