README.md: Link to flatpak delta spec and code

It's important to know where this is used.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

README.md

@@ -1,5 +1,4 @@
-tar-diff
-==
+# tar-diff
 
 `tar-diff` is a golang library and set of commandline tools to diff and patch tar files.
 
@@ -25,7 +24,12 @@ Delta compression is based on [bsdiff](http://www.daemonology.net/bsdiff/) and [
 
 The tar-diff file-format is described in [file-format.md](file-format.md)
 
-License
--
+## Known users
+
+- [Flatpak OCI deltas](https://github.com/cgwalters/flatpak-oci-specs/blob/main/image-deltas.md)
+- [Flatpak OCI processing code](https://github.com/flatpak/flatpak/blob/66b038e14809bc973d46e8078a070dc32e894903/common/flatpak-oci-registry.c#L1689)
+
+## License
+
 tar-diff is licensed under the Apache License, Version 2.0. See
 [LICENSE](LICENSE) for the full license text.