Add version and bug number to 5238 changes file

changes/bug5238

@@ -1,3 +1,4 @@
  o Minor bugfixes:
    - Downgrade "Got a certificate, but we already have it" log mesages from
-     warning to info, except when we're a dirauth.
+     warning to info, except when we're a dirauth. Fixes bug #5238; bugfix
+     on 0.2.1.7-alpha.