docs: add contributing guidelines

Author: dinakars777

CONTRIBUTING.md

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+# Contributing to git-time-machine
+
+Thanks for wanting to contribute! Here are a few guidelines to keep things smooth.
+
+## Before You Start
+
+- Check the [Issues](https://github.com/dinakars777/git-time-machine/issues) page for open tasks. Issues labeled `good first issue` are great starting points.
+- Comment on the issue you want to work on so others know it's taken.
+
+## Pull Request Guidelines
+
+- **One issue per PR.** Keep PRs focused on a single feature or fix. If you want to tackle multiple issues, open separate PRs for each.
+- **Link the issue.** Include `Closes #<number>` in your PR description so the issue gets closed automatically when merged.
+- **Rebase on latest main.** Before opening your PR, make sure your branch is up to date with `main` to avoid merge conflicts.
+
+## Code Quality
+
+- Run `cargo check` before submitting to make sure everything compiles.
+- Run `cargo fmt` to keep formatting consistent.
+- Run `cargo clippy` to catch common mistakes.
+- If you're adding a new dependency to `Cargo.toml`, explain why it's needed in the PR description.
+
+## What Happens After You Submit
+
+- CI will automatically run `cargo check`, `cargo build`, and `cargo test` on your PR.
+- If CI fails, check the logs and push a fix.
+- Once reviewed and approved, your PR will be squash-merged into `main`.
+
+## Questions?
+
+Open a [Discussion](https://github.com/dinakars777/git-time-machine/discussions) or comment on the relevant issue. Happy hacking!