docs: archive project and add migration guide to OTP json

README.md

@@ -1,4 +1,7 @@
 # njson
+
+> **Archived**: This library is no longer maintained. Since OTP 27, the `json` module is included in the standard library and covers the same functionality. We recommend migrating to `json` for all new and existing projects. See the [Migration Guide](#migrating-to-otp-json) below.
+
 [![njson](https://github.com/nomasystems/njson/actions/workflows/ci.yml/badge.svg)](https://github.com/nomasystems/njson/actions/workflows/ci.yml)
 
 `njson` is a robust and efficient Erlang library that seamlessly encodes and decodes JSON
@@ -103,6 +106,95 @@ Encoder:
 --------------------------------------------------------------------------------------
 ```
 
+## Migrating to OTP json
+
+OTP 27 introduced the `json` module, which provides native JSON encoding and decoding. This section covers the key differences and common pitfalls when migrating from `njson`.
+
+### Decoding
+
+`njson:decode/1` returns `{ok, Term}` or `{error, Reason}`. `json:decode/1` returns the term directly and throws on invalid input.
+
+```erl
+%% njson
+{ok, Map} = njson:decode(Binary).
+
+%% json
+Map = json:decode(Binary).
+```
+
+If you relied on `{error, _}` to handle invalid JSON, wrap the call in a try-catch:
+
+```erl
+%% njson
+case njson:decode(Binary) of
+    {ok, Map} -> handle(Map);
+    {error, _} -> handle_error()
+end.
+
+%% json
+try json:decode(Binary) of
+    Map -> handle(Map)
+catch
+    _:_ -> handle_error()
+end.
+```
+
+### Encoding
+
+`njson:encode/1` returns `{ok, Binary}`. `json:encode/1` returns **iodata** (not a binary).
+
+```erl
+%% njson
+{ok, Binary} = njson:encode(Map).
+
+%% json (returns iodata)
+IoData = json:encode(Map).
+```
+
+If you need a `binary()`, wrap the result:
+
+```erl
+Binary = iolist_to_binary(json:encode(Map)).
+```
+
+If you are writing to a socket, file, or HTTP response body, iodata works directly and the conversion is unnecessary.
+
+### Encoding differences
+
+`json:encode/1` does not accept the atom `undefined`. It only handles `true`, `false`, and `null` as atom values. If your maps may contain `undefined` values, strip them before encoding:
+
+```erl
+strip_undefined(Map) when is_map(Map) ->
+    maps:filtermap(
+        fun
+            (_K, undefined) -> false;
+            (_K, V) when is_map(V) -> {true, strip_undefined(V)};
+            (_K, _V) -> true
+        end,
+        Map
+    ).
+
+json:encode(strip_undefined(MyMap)).
+```
+
+### Error handling summary
+
+| Scenario | njson | json (OTP) |
+| --- | --- | --- |
+| Decode success | `{ok, Term}` | `Term` |
+| Decode failure | `{error, Reason}` | Throws exception |
+| Encode success | `{ok, Binary}` | `IoData` |
+| Encode failure | `{error, Reason}` | Throws exception |
+
+### Quick reference
+
+| njson | json (OTP) | Notes |
+| --- | --- | --- |
+| `{ok, Map} = njson:decode(Bin)` | `Map = json:decode(Bin)` | Throws on invalid input |
+| `{ok, Bin} = njson:encode(Map)` | `Bin = iolist_to_binary(json:encode(Map))` | Or use iodata directly |
+| `{ok, IoList} = njson:encode(Map, true)` | `IoData = json:encode(Map)` | Already iodata |
+| `case njson:decode(Bin) of {error,_} -> ...` | `try json:decode(Bin) catch _:_ -> ...` | Error handling |
+
 ## Support
 
 Any doubt or suggestion? Please, check out [our issue tracker](https://github.com/nomasystems/njson/issues).