diff --git a/README.md b/README.md
index bc17884..6006a46 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 This is a nodejs [ZIM](http://www.openzim.org/wiki/OpenZIM) file creator -- mostly a drop-in replacement for [zimwriterfs](https://github.com/wikimedia/openzim/tree/master/zimwriterfs).
 
-A notable exception: it does't support *withFullTextIndex* option.
+A notable exception: it does't support *withFullTextIndex* option (as index format is not documented at the [OpenZIM wiki](http://www.openzim.org/wiki/ZIM_Index_Format)).
 
 ## Installation
 Requirement: `node` version >=6.x