[glib: 1/2] docs: Mention the stable/unstable support version in README.md

[Simon McVittie <smcv src gnome org>]

commit e2d1ea3b9e1acc462d6aff2f5f65deb16c19e91a
Author: Philip Withnall <pwithnall endlessos org>
Date:   Fri Jun 18 10:43:23 2021 +0100

    docs: Mention the stable/unstable support version in README.md
    
    Signed-off-by: Philip Withnall <pwithnall endlessos org>

 README.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/README.md b/README.md
index c53dd6f4d..df480d1d0 100644
--- a/README.md
+++ b/README.md
@@ -15,6 +15,14 @@ The official web site is:
 
 See the file '[INSTALL.in](INSTALL.in)'
 
+## Supported versions
+
+Only the most recent unstable and stable release series are supported. All
+older versions are not supported upstream and may contain bugs, some of
+which may be exploitable security vulnerabilities.
+
+See [SECURITY.md](SECURITY.md) for more details.
+
 ## How to report bugs
 
 Bugs should be reported to the GNOME issue tracking system.