[python-matplotlib] add link to upstream libpng 1.5 issue

[dmalcolm]

commit 70576db6a2d7e94f214198f967c44804691a7c85
Author: David Malcolm <dmalcolm at redhat.com>
Date:   Tue Dec 6 17:04:00 2011 -0500

    add link to upstream libpng 1.5 issue

 python-matplotlib.spec |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/python-matplotlib.spec b/python-matplotlib.spec
index efdf300..a02e940 100644
--- a/python-matplotlib.spec
+++ b/python-matplotlib.spec
@@ -42,6 +42,7 @@ Patch2:		0001-Bugfix-propagate-timezone-info-in-plot_date-xaxis_da.patch
 Patch3:         matplotlib-1.0.1-tkinter.patch
 
 # Fix building against libpng 1.5
+#   https://github.com/matplotlib/matplotlib/issues/234
 # Based on:
 #   https://github.com/matplotlib/matplotlib/commit/45c46672648e3b4a277bf7ff42b3baf56a98bcec
 Patch4:         use-png-accessor-functions.patch