Issue #13989: Document that GzipFile does not support text mode.

Also, give a more helpful error message when opened with an invalid mode string.
diff --git a/Misc/NEWS b/Misc/NEWS
index da90e9e..f45da75 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -113,6 +113,9 @@
 Library
 -------
 
+- Issue #13989: Document that GzipFile does not support text mode, and give a
+  more helpful error message when opened with an invalid mode string.
+
 - Issue #13590: On OS X 10.7 and 10.6 with Xcode 4.2, building
   Distutils-based packages with C extension modules may fail because
   Apple has removed gcc-4.2, the version used to build python.org