[gnome-games] hacking: Make tag rules more explicit

[Adrien Plazas <aplazas src gnome org>]

commit 92f148ef429894874dbfce18ed1b3c0157d76a42
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Thu Feb 9 10:15:09 2017 +0100

    hacking: Make tag rules more explicit

 HACKING |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/HACKING b/HACKING
index fb7b319..de19eb8 100644
--- a/HACKING
+++ b/HACKING
@@ -23,6 +23,13 @@ General rules:
   when reading we know the name refers to an operation,
 - write property names between single quotes and in kebab-case.
 
+The tag can be:
+- the name of the sub-directory of data, plugins or src in which the
+  change was made,
+- 'build' if it affects the build system,
+- anything else that seems more appropriate for the change,
+- nothing if multiple parts were affected.
+
 The actual changes:
 - the first paragraph explains what the commit does from an objective
   point of view: the code change,