Add note about linked lists data type choice for edn::Value

edn/src/types.rs

@@ -33,6 +33,9 @@ pub enum Value {
     Keyword(symbols::Keyword),
     NamespacedKeyword(symbols::NamespacedKeyword),
     Vector(Vec<Value>),
+    // We're using a LinkedList here instead of a Vec or VecDeque because the
+    // LinkedList is faster for appending (which we do a lot of).
+    // See https://github.com/mozilla/mentat/issues/231
     List(LinkedList<Value>),
     // We're using BTree{Set, Map} rather than Hash{Set, Map} because the BTree variants
     // implement Hash. The Hash variants don't in order to preserve O(n) hashing