Document lookup-ref differences.

Differences-from-Datomic.md

@@ -75,4 +75,10 @@ Datomish's JS API exposes a simple schema interface through `db.ensureSchema`. T
      "doc":          "A snapshot of the page's content. Should be plain text."},
   ]
 }
-```
+```
+
+## Lookup Refs
+
+There's a small syntactic difference with how lookup refs are indicated in EDN. Where in Datomic they will be indicated as `[:namespace/attr value]`, in Mentat they are represented as `(lookup-ref :namespace/attr value)`.
+
+This may be fixed in a future version of Mentat, see issue https://github.com/mozilla/mentat/issues/392.