[glib/mainloop-id-rationale] Document the stance on ID-based mainloop APIs

[Matthias Clasen <matthiasc src gnome org>]

commit 74f9f5d47d2894bdfdfa55668f44d3ea0b88e995
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jul 27 10:21:43 2021 -0400

    Document the stance on ID-based mainloop APIs
    
    This will make it easier to reject MRs like
    https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2199
    in the future.

 README.rationale | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/README.rationale b/README.rationale
index 85a3dfd59..e1d459376 100644
--- a/README.rationale
+++ b/README.rationale
@@ -8,3 +8,9 @@ giving a brief rationale of each decision, plus a link to further discussion.
    already supported by GLib and GNOME tools, and accomplish the same task as
    compiler attributes. GLib does not provide macros for attributes like
    nonnull because it would not use them.
+
+ * Main loop API:
+
+   The ID-based mainloop APIs (g_idle_add, g_timeout_add, etc) are considered
+   legacy, and new features (such as g_source_set_static_name) will only be
+   added to the explicit GSource APIs.