explain the new pip freeze in the coverage commands

tox.ini

@@ -54,6 +54,11 @@ commands =
 [testenv:coverage]
 # coverage (with --branch) takes about 65% longer to run
 commands =
+	 # As an aid to debugging, dump all of the Python packages and their
+	 # versions that are installed in the test environment.  This is
+	 # particularly useful to get from CI runs - though hopefully the
+	 # version pinning we do limits the variability of this output
+	 # somewhat.
          pip freeze
          tahoe --version
          coverage run --branch -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:allmydata}