docs: add CHANGELOG and CONTRIBUTING

[rellampec]

Introduce a CHANGELOG (reconstructed from merged PRs and release tags back to 0.23.0, with an Unreleased section) and a CONTRIBUTING guide documenting the fork -> topic-branch -> test (Minitest) -> CHANGELOG -> PR flow.

Upstream master previously had neither file.

What this does

Adds two governance/docs files the repo currently lacks — no code or behaviour changes.

CHANGELOG.md (new)

The project has no changelog today; releases are only discoverable via tags and merged PRs. This reconstructs one from
the merged pull requests and release tags back to 0.23.0, and opens an ## Unreleased section for future
entries.

• Header notes that the file was introduced after 0.26.0, so pre-0.26.0 entries are reconstructed and may be
  incomplete.
• #60 (SDL schema loading) is placed under
  Unreleased — it's merged but not yet in a tagged release — rather than under 0.26.0.
• Older versions point to the release tags.

CONTRIBUTING.md (new)

A short contribution guide matching how the project actually works:

• Fork → bundle install → bundle exec rake (the default task runs the test suite and RuboCop).
• Makes the Minitest convention explicit (the suite is Minitest, not RSpec) so new contributors don't introduce a
  second framework.
• Topic-branch-off-master, write tests (characterization tests around the parser/fragment handling are called out as
  welcome), update the Unreleased changelog section, open a PR against master.

Why

Lowers the barrier for new contributors and gives a single place to record changes. This is the first of a small
series of focused PRs — it's the base the documentation PR for inline named fragments builds on.

Notes for reviewers

• Pure docs: CHANGELOG.md and CONTRIBUTING.md only — no lib/ or test/ changes.
• The reconstructed pre-0.26.0 history is best-effort; happy to adjust any attribution or wording.
• If you'd prefer a different changelog format (e.g. Keep a Changelog headings), say the word and I'll reformat.