diff --git a/README.md b/README.md
index be0dfe2..90bf5e8 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,17 @@
 # Microsoft Selawik
 Selawik is an open source replacement for Segoe UI. 
 
-# Known open issues:
+[Microsoft's Open Source Fonts](https://github.com/Microsoft/fonts)
+
+## Known open issues:
 * Selawik is missing kerning to match Segoe UI. 
 * Selawik needs improved hinting. 
 
-[Microsoft's Open Source Fonts](https://github.com/Microsoft/fonts)
+## File format
+The original multiple-master source files are in the [Glyphs][] editor format.
+Conversion to UFO is provided with designspace files that define the
+interpolation points. If you don't have access to Glyphs, try using
+[GlyphsLib][] for interconversion.
+
+  [Glyphs]: https://glyphsapp.com
+  [GlyphsLib]: https://github.com/googlefonts/glyphsLib