[retro-gtk] hacking: Document the documentation style

[Adrien Plazas <aplazas src gnome org>]

commit a6a3f6d526c793a6d910514801642dadbbc5e36c
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Tue Oct 27 07:51:07 2020 +0100

    hacking: Document the documentation style

 HACKING.md | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 5718ce8b..21abcba3 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -544,6 +544,38 @@ FooBaz *baz = foo_bar_generate_baz (bar);
 g_object_unref (baz);
 ```
 
+## Documentation Style
+
+### XML Documentation
+
+This describes how to write XML documentation like the reference manual.
+
+Indent with two spaces.
+
+Start paragraphs on a new line, indented.
+Break lines in paragraphs after 80 characters when possible, including the
+indentation.
+Start sentences on a new line so they can be changed without having to reflow
+the whole paragraph.
+
+Don't break lines in the middle of inlined elements like links, only before or
+after.
+It is acceptable to let the element exceeds the 80 characters limit if you have
+no other choice, e.g. if a URL you are linking to is too long.
+
+### MarkDown Documentation
+
+This describes how to write MarkDown documentation like README.md or HACKING.md.
+
+Break lines in paragraphs after 80 characters when possible.
+Start sentences on a new line so they can be changed without having to reflow
+the whole paragraph.
+
+Don't break lines in the middle of inlined elements like links, only before or
+after.
+It is acceptable to let the element exceeds the 80 characters limit if you have
+no other choice, e.g. if a URL you are linking to is too long.
+
 ## Commit Message Style
 
 Commit messages have 4 parts: a tag, a short explanation, a long explanation,