wrote loading xyz docs

package/doc/sphinx/source/documentation_pages/loading_files/xyz.rst

@@ -4,3 +4,22 @@
 Loading XYZ files with MDAnalysis
 #################################
 
+MDAnalysis is able to read and write the ubiquitous XYZ format to provide
+a basic topology and trajectory.  The format follow matches the
+`VMD xyzplugin`_ and assumes that columns are whitespace delimited.
+
+.. _`VMD xyzplugin`: http://www.ks.uiuc.edu/Research/vmd/plugins/molfile/xyzplugin.html
+
+When used as a topology the atom names will be read from the file with the
+atom types and masses guessed.  Default resids and segids of ``1`` and ``SYSTEM``
+will be assigned to all atoms.
+For implementation details see :mod:`MDAnalysis.topology.XYZParser`.
+
+When used as a trajectory MDAnalysis will read multiple frames of coordinates.
+It is assumed that all frames have (n_atoms + 2) lines.
+For implementation details see :mod:`MDAnalysis.coordinates.XYZ`.
+
+.. Note
+   The XYZ format provides no box information, any Universe created from
+   XYZ files will have this information set to 0.
+