diff --git a/docs/ics_format.txt b/docs/ics_format.txt
index 8b99bcb..4becc78 100644
--- a/docs/ics_format.txt
+++ b/docs/ics_format.txt
@@ -1,3 +1,8 @@
+RFC5545
+-------
+https://www.rfc-editor.org/rfc/rfc5545
+
+
 ICS files from Microsoft Outlook differ from those that were created by Gnome Evolution.
 (semicolons are used instead of colons at some position)