[gnome-games] hacking: Add UI guidelines

[Adrien Plazas <aplazas src gnome org>]

commit ee9fc282d0bf07d9c796bfe56c23e7da4f05d832
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon Jan 30 09:47:08 2017 +0100

    hacking: Add UI guidelines
    
    This will help newcomers to understand how the UI is organized.

 HACKING |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)
---
diff --git a/HACKING b/HACKING
index c066e22..fb7b319 100644
--- a/HACKING
+++ b/HACKING
@@ -36,3 +36,16 @@ The reasoning behind the changes:
   point of view: the behavior change,
 - don't write about what was before this commit, write about what this
   commit does.
+
+UI Guidelines
+=============
+
+The UI is splitted into pages, each page consists of two widgets:
+- the header bar named FooHeaderBar,
+- the body of the window named FooBox.
+Any possible state of the page foo must be represented by FooHeaderBar
+and FooBox and they can't be contained by any other widget than the
+window.
+
+This allows consistency and helps to quickly know which widget
+represents what.