[mutter] README: Clarify how to name flag types

[Marge Bot <marge-bot src gnome org>]

commit dcbd96df38f69b10e6ad9eb213c89f5193240381
Author: Jonas Ådahl <jadahl gmail com>
Date:   Tue Mar 9 20:40:39 2021 +0100

    README: Clarify how to name flag types
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1765>

 README.md | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index cedaf25b4e..d57296a7c7 100644
--- a/README.md
+++ b/README.md
@@ -63,7 +63,16 @@ with some additions:
    that may have side effects.
 
  - Instead of boolean arguments in functions, prefer enums or flags when
-   they're more expressive.
+   they're more expressive. The naming convention for flags is
+
+```c
+typedef _MetaSomeThingFlags
+{
+  META_SOME_THING_FLAG_NONE = 0,
+  META_SOME_THING_FLAG_ALTER_REALITY = 1 << 0,
+  META_SOME_THING_FLAG_MANIPULATE_PERCEPTION = 1 << 1,
+} MetaSomeThingFlags;
+```
 
  - Use `g_new0()` etc instead of `g_slice_new0()`.