docs: adds contribution guidelines (#119)

Contributes to #2 

Adds some general contribution guidelins for this project. I've used
some other testcontainers projects (`java`, `ruby`, `python`, `rust`) as
reference.

---------

Co-authored-by: Artem Medvedev <i@ddtkey.com>

CONTRIBUTING.md

@@ -0,0 +1,68 @@
+# Contributing to testcontainers-rs-modules
+
+First of all, thank you for contributing to testcontainers-rs-modules-community. All contributions are welcome!
+
+## Getting started
+
+### The quick-wins!
+
+- Consider giving a star to this repo and become one of our [stargazers](https://github.com/testcontainers/testcontainers-rs-modules-community/stargazers)
+- Consider joining our [Rust channel](https://testcontainers.slack.com/archives/C048EPGRCER) within the [TestContainers Slack Workspace](https://testcontainers.slack.com/)
+
+### Reporting bugs
+
+- Check if there is an [existing issue](https://github.com/testcontainers/testcontainers-rs-modules-community/issues) first. 
+- You may also want to check bugs in [testcontainers-rs library](https://github.com/testcontainers/testcontainers-rs/issues) when applicable
+- When in doubt whether you've found new issue/bug or not, consider discussing it with our community on Slack
+
+### Requesting new modules
+
+- Check if there is an [existing issue](https://github.com/testcontainers/testcontainers-rs-modules-community/issues), since your request might be tracked there.
+- Feel free to reach out to our community to clarify your use case!
+
+## Code Contributions
+
+### Setting up local development
+
+- Ensure you have an [up-to-date Rust toolchain](https://rustup.rs/), with `clippy` and `rustfmt` components installed
+- Install the [cargo-hack](https://github.com/taiki-e/cargo-hack) subcommand (recommended)
+- Fork this repository
+
+### Working with existing modules
+
+- When extending/changing existing API, ensure that any new ones are tracked on [rustdocs](https://docs.rs/testcontainers-modules/latest/testcontainers_modules/)
+- When fixing an issue, consider providing a new test case for it
+
+### Adding new modules or changing existing ones
+
+Before adding a new module, it's recommended reviewing the
+[testcontainers-rs library](https://github.com/testcontainers/testcontainers-rs)
+along with existing modules that are built on top of it! In addition, pay attention to our project conventions : public APIs 
+are exposed through `mod.rs` files and conditionally compiled as Cargo features. 
+
+- Ensure you have a proper feature configuration on `Cargo.toml` when adding a new module
+- Ensure you have declared pinned versions of Docker image tags for your module
+- Consider providing a `Default` trait implementation for your `TestContainer` struct when applicable
+- Consider also using the `Builder` pattern when your `TestContainer` accepts several different env vars!
+- Ensure you have added proper `rustdocs` reflecting the image reference (e.g. to [docker-hub](hub.docker.com)) and examples of how to use your APIs.
+- Ensure you have added tests exercising your module
+- Consider also providing an example on how to use your module from an app
+
+### Raising Pull Requests
+
+- Ensure you'll have a green build on your commits, for instance running some of our CI checks
+
+```bash
+cargo fmt --all -- --check
+cargo clippy --all-features
+cargo hack test --each-feature --clean-per-run 
+```
+- Consider following [conventional commits](https://julien.ponge.org/blog/the-power-of-conventional-commits/) when adding commits (recommended)
+- Raise your PR 🔥
+- Ensure you follow the [conventional commits](https://julien.ponge.org/blog/the-power-of-conventional-commits/) in your Pull Request title
+- Don't forget to [link an existing issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) when applicable (fixing it or just mentioning it)
+
+## License
+
+Please note that all code contributed by you will follow the [MIT license](http://opensource.org/licenses/MIT),
+without any additional terms.