Add source file format in documentation.

docs/source/devguide/index.rst

@@ -17,6 +17,7 @@ as debugging.
     workflow
     xml-parsing
     statepoint
+    source
     particle_restart
     track
     voxel

docs/source/devguide/source.rst

@@ -0,0 +1,21 @@
+.. _devguide_source:
+
+==================
+Source File Format
+==================
+
+Normally, source data is stored in a state point file. However, it is possible
+to request that the source be written separately, in which case the format used
+is that documented here.
+
+**/filetype** (*int*)
+
+    Flags what type of file this is. A value of -1 indicates a statepoint file,
+    a value of -2 indicates a particle restart file, a value of -3 indicates a
+    source file, and a value of -4 indicates a track file.
+
+**/source_bank** (Compound type)
+
+    Source bank information for each particle. The compound type has fields
+    ``wgt``, ``xyz``, ``uvw``, and ``E`` which represent the weight, position,
+    direction, and energy of the source particle, respectively.