Clarify purpose of cextern directory.
Clarify purpose of cextern directory.

keflavich/APEX_CMZ_H2CO,astropy/ccdproc,evertrol/ccdproc,pulsestaysconstant/ccdproc,crawfordsm/ccdproc,alexrudy/Zeeko,indiajoe/ccdproc,allisony/aplpy,mwcraig/ccdproc,mwcraig/aplpy,keflavich/APEX_CMZ_H2CO,alexrudy/Zeeko,adamginsburg/APEX_CMZ_H2CO,adamginsburg/APEX_CMZ_H2CO