[libdazzle] update CONTRIBUTING.md

[Christian Hergert <chergert src gnome org>]

commit 31d02939590759686e4d8c0df7b90a1e0b6eeccb
Author: Christian Hergert <chergert redhat com>
Date:   Sat Jun 17 14:46:22 2017 -0700

    update CONTRIBUTING.md

 CONTRIBUTING.md |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 9537f43..1fcc2e7 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -14,6 +14,15 @@ It is our belief that you should always retain copyright on your own work.
 When working on a new widget or tool, try to write unit tests to prove the implementation.
 Not everything we have in the code base has tests, and ideally that will improve, not get worse.
 
+## Troubleshooting
+
+If you configure the meson project with `-Denable_tracing=true` then libdazzle with be built with tracing.
+This allows various parts of the code to use `DZL_ENTRY`, `DZL_EXIT` and other tracing macros to log 
function calls.
+You might find this useful in tracking down difficult re-entrancy or simply learn "how does this work".
+
+If you need to add additional tracing macros to debug a problem, it is probably a good idea to submit a 
patch to add them.
+Chances are someone else will need to debug stuff in the future.
+
 ## Code Style
 
 We follow the GObject and Gtk coding style.