[devhelp] hacking: add link to blog post for more info

[Sébastien Wilmet <swilmet src gnome org>]

commit 51716a1fe5b0db45e3d54054d8e7bb471edfea47
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Aug 25 05:44:49 2019 +0200

    hacking: add link to blog post for more info

 HACKING | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/HACKING b/HACKING
index 33d32df7..737f94d3 100644
--- a/HACKING
+++ b/HACKING
@@ -29,7 +29,7 @@ classes for the containment hierarchy. With the goal to create a software
 product line [2] for Devhelp: keep the Devhelp application generic, and create
 a new application specific to GNOME, with both applications having a really
 small codebase (the GNOME-specific features would be written as a configurable
-library as well).
+library as well). For more information see also [3].
 
 [1] “a corpus of functionality built as a set of libraries that can be sliced
     and remixed in different ways per the needs of different use-cases”
@@ -38,6 +38,8 @@ library as well).
 
 [2] https://www.amazon.com/Feature-Oriented-Software-Product-Lines-Implementation/dp/3642375200/
 
+[3] https://swilmet.be/blog/2019/08/18/10-years-later/
+
 C code conventions
 ------------------