Create contributing.md (#48)

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Author: NobodyXu

contributing.md

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+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0
+   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license
+   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+### Run integration tests
+
+Requires `docker` and [`cargo-hack`].
+
+Check [getting Docker guide](https://docs.docker.com/get-docker/) on how to install docker,
+and use `cargo install cargo-hack` to install [`cargo-hack`].
+
+```
+./run_ci_tests.sh
+```
+
+It will create a container which runs sshd, set up an ssh-agent, and set environment variables
+that are required to run the integration tests.
+It will also test different combination of feature flags to ensure they all compile without error.
+
+[`cargo-hack`]: https://github.com/taiki-e/cargo-hack