Added Amazon Labs’ doc about ION

ReadingSystems/Kindle/KDF-KFX/KindlePreviewer3.md

@@ -78,6 +78,7 @@ When converting a file with Kindle Previewer, which takes some time (yeah, that
 
 - the link to the external stylesheets is erased (`head`);
 - book styles are parsed and inlined to each tag (`style` attribute);
+- the book is entirely rendered in phantom.js in order to compute styles;
 - computed styles are then inlined as well (`computedstyle` attribute).
 
 You end up with something like this: 
@@ -104,6 +105,8 @@ As regards drop caps, Amzn has created its own non-standard style properties whi
 
 And now to the **ion** data representation…
 
+[Here is Amazon Labs’ documentation.](https://amznlabs.github.io/ion-docs/)
+
 I don’t really want to make it obscure so if you want further details, check [this post](http://www.mobileread.com/forums/showpost.php?p=3269649&postcount=360).
 
 You can also check `ionnames.csv` ([link](https://github.com/FriendsOfEpub/WillThatBeOverriden/blob/master/ReadingSystems/Kindle/KDF-KFX/ionnames.csv)). You’ll see `epub:type` and CSS properties + values in there so maybe we could build a list of supported styles from here.