chore: Add a changelog (close #118)

Also add a workflow to enforce it is updated on pull requests.

Author: peso

.github/workflows/changelog.yml

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+name: Check for Changelog entries
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review, labeled, unlabeled]
+
+jobs:
+  # Enforces the update of a changelog file on every pull request 
+  changelog:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: dangoslen/changelog-enforcer@v3

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+
+## [unreleased] - Next Date
+
+### Added
+
+- A CHANGELOG.md file.
+- Details to the SVG output.
+- Option to specify revset to render a subset of the graph.
+- Gitea merge pattern.
+
+### Changed
+
+- Add clippy warning for complex functions.
+- Code refactoring to make it easier to understand.
+
+### Removed
+
+- Remove pager
+
+
+## [0.7.0] - 2025-11-14
+
+Last release where library is part of git-graph.
+
+### Added
+
+- (BREAKING) graph::get_repo, add argument skip_repo_owner_validation
+  false gives the previous behaviour.
+- (BREAKING) GitGraph::new, add argument start_point to control where
+  traversal should start.
+  Set to None to get the previous behaviour.
+  
+- Lots of API docs
+- "trunk" as supported main branch name
+
+### Changed
+
+- Update git2 dependency to version 0.20
+
+### Removed
+
+- (BREAKING) GitGraph public fields "tags" and "branches"
+
+
+## [0.6.0] - 2024-05-24
+
+### Added
+
+- Reverse order option