PEP 508: Add canonical spec link (#4277)

Author: mhsmith

peps/pep-0508.rst

@@ -10,6 +10,8 @@ Created: 11-Nov-2015
 Post-History: 05-Nov-2015, 16-Nov-2015
 Resolution: https://mail.python.org/pipermail/distutils-sig/2015-November/027868.html
 
+.. canonical-pypa-spec:: :ref:`packaging:dependency-specifiers`
+
 
 Abstract
 ========