add section on missing functionality

README.md

@@ -4,7 +4,7 @@ An efficient 64-bit integer with correct overflow.
 
 Bitwise operators in javascript can only use 32 bits. Sometimes, external protocols use 64-bit integers. This package implementes such integers with "correct" overflow behavior.
 
-This is a low-level package focussed on speed. The 64-bit integers are represented as a 2-tuple of 32-bit numbers.
+This is a low-level package focussed on speed. The 64-bit integers are represented as a 2-tuple of 32-bit numbers. There are helpers for conversion to string and `elm/bytes` `Bytes`.
 
 ```elm
 import Int64 
@@ -33,3 +33,25 @@ Performance is roughly half for subtraction, addition and the bitwise operators.
 ## Reliabiltiy
 
 This package is extensively tested with fuzz tests. Nonetheless, the logic is extremely tricky, so there might still be bugs.
+
+## Missing functionality
+
+
+1. `Int64 -> Int`
+
+    This is unsafe, because `Int` cannot store all the information of a `Int64`. If you think the conversion is safe for you, use: 
+
+    ```elm
+    Int64.fromInt 42
+        |> Int64.toSignedString
+        |> String.fromInt
+        --> Just 42
+    ```
+
+2. `String -> Int64`
+
+    And by extension converting 64-bit integers in e.g. JSON into `Int64`. This is tricky to implement, but PRs are welcome! 
+
+3. Multiplication, integer division, modulo
+
+    Would be nice to have, again PRs are welcome!