docs: add CHANGELOG and CONTRIBUTING

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project are documented in this file.
+
+This file was introduced after the 0.26.0 release; entries for earlier versions are
+reconstructed from the merged pull requests and release tags and may be incomplete.
+
+## Unreleased
+
+* [#60](https://github.com/github-community-projects/graphql-client/pull/60): Allow loading the schema from an SDL definition file (`.graphql` / `.graphqls`) in addition to JSON - [@Tabby](https://github.com/Tabby).
+* Your contribution here.
+
+## 0.26.0 (2025-05-29)
+
+* Maintenance release: CI workflow permissions, test fixes, and dependency updates ([#69](https://github.com/github-community-projects/graphql-client/pull/69), [#70](https://github.com/github-community-projects/graphql-client/pull/70), [#71](https://github.com/github-community-projects/graphql-client/pull/71)).
+* [#66](https://github.com/github-community-projects/graphql-client/pull/66): Tidy Dependabot configuration - [@issyl0](https://github.com/issyl0).
+* Dependency and GitHub Actions bumps ([#47](https://github.com/github-community-projects/graphql-client/pull/47), [#58](https://github.com/github-community-projects/graphql-client/pull/58), [#64](https://github.com/github-community-projects/graphql-client/pull/64)).
+
+## 0.25.0 (2024-12-12)
+
+* [#44](https://github.com/github-community-projects/graphql-client/pull/44): Contribution from [@ajsharma](https://github.com/ajsharma).
+* Dependency and CI maintenance ([#30](https://github.com/github-community-projects/graphql-client/pull/30), [#41](https://github.com/github-community-projects/graphql-client/pull/41), [#42](https://github.com/github-community-projects/graphql-client/pull/42)).
+
+## 0.24.0 (2024-11-13)
+
+* [#43](https://github.com/github-community-projects/graphql-client/pull/43): Allow accessing the full GraphQL response - [@billybonks](https://github.com/billybonks).
+
+## 0.23.0 (2024-06-17)
+
+* [#28](https://github.com/github-community-projects/graphql-client/pull/28): Fix possible-types resolution - [@rmosolgo](https://github.com/rmosolgo).
+* Dependency and CI maintenance ([#24](https://github.com/github-community-projects/graphql-client/pull/24), [#25](https://github.com/github-community-projects/graphql-client/pull/25), [#26](https://github.com/github-community-projects/graphql-client/pull/26)).
+
+## Earlier releases
+
+See the [release tags](https://github.com/github-community-projects/graphql-client/tags) for versions 0.22.0 and earlier.

CONTRIBUTING.md

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+# Contributing to graphql-client
+
+This project is the work of [many contributors](https://github.com/github-community-projects/graphql-client/graphs/contributors).
+You're encouraged to submit [pull requests](https://github.com/github-community-projects/graphql-client/pulls)
+and to [propose features and discuss issues](https://github.com/github-community-projects/graphql-client/issues).
+
+In the examples below, substitute your GitHub username for `contributor` in URLs.
+
+## Fork the Project
+
+Fork the [project on GitHub](https://github.com/github-community-projects/graphql-client) and check out your copy.
+
+```
+git clone https://github.com/contributor/graphql-client.git
+cd graphql-client
+git remote add upstream https://github.com/github-community-projects/graphql-client.git
+```
+
+## Bundle Install and Test
+
+Ensure that you can build the project and run the tests.
+
+```
+bundle install
+bundle exec rake
+```
+
+The default Rake task runs the test suite **and** RuboCop. The project uses
+**Minitest** (not RSpec) — keep new tests in that framework and style, alongside the
+existing `test/test_*.rb` files.
+
+## Create a Topic Branch
+
+Keep your fork up to date and create a topic branch off `master` for your change.
+
+```
+git checkout master
+git pull upstream master
+git checkout -b my-feature-branch
+```
+
+## Write Tests
+
+Add Minitest coverage for any behavior you change or add. Characterization tests that
+pin existing behavior are especially welcome around the parser and fragment handling.
+Run the full suite and RuboCop before pushing:
+
+```
+bundle exec rake
+```
+
+## Update the CHANGELOG
+
+Add a line describing your change to the **Unreleased** section of `CHANGELOG.md`,
+crediting yourself:
+
+```
+* [#123](https://github.com/github-community-projects/graphql-client/pull/123): Short description - [@contributor](https://github.com/contributor).
+```
+
+## Commit and Push Your Changes
+
+Use clear commit messages. Keep the history focused — one logical change per branch.
+
+```
+git commit -am "Add my feature"
+git push origin my-feature-branch
+```
+
+## Open a Pull Request
+
+[Open a pull request](https://github.com/github-community-projects/graphql-client/compare)
+from your topic branch against `master`. Describe the motivation and the change, and link
+any related issues.
+
+## Be Patient
+
+Maintainers review on a best-effort basis. It's likely a reviewer will ask for changes —
+that's a normal part of getting a contribution merged. Thanks for contributing!