docs: add CHANGELOG and LICENSE to mix docs and package files

- add MIT LICENSE file (declared in package but missing from repo)
- include CHANGELOG.md and LICENSE in docs() extras
- include LICENSE in package files list so it ships to Hex
- skip undefined-reference warnings for CHANGELOG.md (release notes
  reference @doc false internals like Config.refresh_access_token/2)

Author: Longbridge Dev

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 gilbertwong96
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

mix.exs

@@ -39,9 +39,10 @@ defmodule Longbridge.MixProject do
       main: "readme",
       source_url: "https://github.com/gilbertwong96/longbridge",
       homepage_url: "https://github.com/gilbertwong96/longbridge",
-      extras: ["README.md"],
+      extras: ["README.md", "CHANGELOG.md", "LICENSE"],
       skip_undefined_reference_warnings_on: [
         "README.md",
+        "CHANGELOG.md",
         "lib/longbridge/quote_context.ex",
         # References `Longbridge.OAuth.token_path/1` which is @doc false
         # (a back-compat shim kept for callers that imported it before
@@ -142,7 +143,7 @@ defmodule Longbridge.MixProject do
         "Upstream" => "https://github.com/longbridge/longbridge",
         "Longbridge" => "https://open.longbridge.com"
       },
-      files: ~w[lib protos .formatter.exs mix.exs README.md]
+      files: ~w[lib protos .formatter.exs mix.exs README.md CHANGELOG.md LICENSE]
     ]
   end
 end