give a little history

pyrex/README

 This directory contains a collection of generic Pyrex include files.
 Do not put any pyrex modules in here.
+
+Note: early in Shrapnel's history, we built our own 'libc.pxd' and 'python.pxi' files
+that eventually made their way into the Pyrex/Cython distribution, and have since been
+split into separate 'packages' (e.g., libc.string.memcpy).  What remains here should
+truly be either platform or shrapnel-specific.
+