[gtk] Update contribution guide

[Matthias Clasen <matthiasc src gnome org>]

commit ee26e282ccd019e23620ce31fd4326765f161d68
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Dec 5 11:34:52 2020 -0500

    Update contribution guide
    
    Make my feelings about bug reporting by screencast known.

 CONTRIBUTING.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 474598fc81..80be9420fc 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -56,6 +56,12 @@ If the issue includes a crash, you should also include:
  0. the eventual warnings printed on the terminal
  0. a backtrace, obtained with tools such as GDB or LLDB
 
+It is fine to include screenshots of screen recordings to demonstrate
+an issue that is best to understand visually, but please don't just
+dump screen recordings without further details into issues. It is
+essential that the problem is described in enough detail to reproduce
+it without watching a video.
+
 For small issues, such as:
 
  - spelling/grammar fixes in the documentation