Add comment about enabling pdb in tests

Uncommenting the pdb=1 line will cause pdb to open on every test
failure, allowing immediate debugging of the failure case

setup.cfg

 [nosetests]
 logging-clear-handlers=1
 with-xunit=1
-rednose=1
\ No newline at end of file
+rednose=1
+
+# Uncomment the following line to open pdb when a test fails
+pdb=1
\ No newline at end of file