[gtk/license-clarification: 2/2] Add a license clarification

[Matthias Clasen <matthiasc src gnome org>]

commit 52c82c109857c0978cc0a5d6e3c4064052f36db5
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Sep 1 14:11:28 2020 -0400

    Add a license clarification

 README.md | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/README.md b/README.md
index 50d6a3d9ee..cbe7809f58 100644
--- a/README.md
+++ b/README.md
@@ -161,3 +161,7 @@ version 2.1 or, at your option, any later version, as published by the Free
 Software Foundation.
 
 Please, see the [`COPYING`](./COPYING) file for further information.
+
+GTK includes a small number of source files under the Apache license:
+- A fork of the roaring bitmaps implementation in [gtk/roaring](./gtk/roaring)
+- An adaptation of timsort from python in [gtk/timsort](./gtk/timsort)