Add link to OpenType's composite glyph description.
diff --git a/include/freetype/freetype.h b/include/freetype/freetype.h
index ca4d1cc..a8cea7f 100644
--- a/include/freetype/freetype.h
+++ b/include/freetype/freetype.h
@@ -3997,6 +3997,8 @@ FT_BEGIN_HEADER
* interpreted depending on the flags returned in `*p_flags`. See the
* OpenType specification for details.
*
+ * https://docs.microsoft.com/en-us/typography/opentype/spec/glyf#composite-glyph-description
+ *
*/
FT_EXPORT( FT_Error )
FT_Get_SubGlyph_Info( FT_GlyphSlot glyph,