docs: add CONTRIBUTING.md, CODE_OF_CONDUCT.md (#240)

Author: slvrtrn

CODE_OF_CONDUCT.md

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+### Code of Conduct
+
+ClickHouse Rust client follows the [Rust Code of Conduct](https://www.rust-lang.org/policies/code-of-conduct).
+By participating in the project, you are expected to uphold this code.
+Please report any unacceptable behavior to the project team in the [community Slack](https://clickhouse.com/slack).

CONTRIBUTING.md

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+# Contribution guide
+
+## How to contribute
+
+Please create an issue for discussion before submitting a pull request. This helps to ensure that your contribution
+aligns with the project's goals and standards, and allows maintainers to provide guidance on the implementation details.
+
+When submitting a pull request, please ensure that it includes:
+
+- A clear description of the changes made and the motivation behind them, as well as the link to the related issue.
+- Unit and integration tests for the new functionality or bug fixes, if applicable.
+- Documentation and examples updates, if the changes affect the public API or usage of the crate.
+- If the pull request is a work in progress, please mark it as such and indicate what is still needed to complete it.
+
+## Upgrading dependencies
+
+For security reasons, pull requests from external contributors that upgrade dependencies will not be accepted, unless a
+particular change is discussed in advance with the project maintainers.
+
+## Development workflow
+
+All commands assumed to be run from the root of the repository.
+
+### Environment
+
+- A fairly recent version of Rust, see `rust-version` in [Cargo.toml](./Cargo.toml).
+- Docker compose or a running ClickHouse server, accessible at `http://localhost:8123` with the `default` user and no
+  password.
+
+### Start ClickHouse
+
+```sh
+docker compose up -d
+```
+
+### Running tests
+
+To run all tests, you could use one of the following commands:
+
+```sh
+cargo test
+cargo test --no-default-features
+cargo test --all-features
+```
+
+See also: [CI test commands](.github/workflows/ci.yml)
+
+### Documentation
+
+If you add a public API, please document it, as at a certain point the crate documentation will be enforced by the
+`missing_docs` lint rule. Additionally, consider adding runnable examples to the [examples directory](./examples)
+and to the [examples overview](./examples/README.md).
+
+Checking documentation can be done with:
+
+```sh
+cargo doc --all-features
+```
+
+### Lint
+
+It is recommended to run Clippy before submitting a pull request. For example:
+
+```sh
+cargo clippy --all-targets --no-default-features
+cargo clippy --all-targets --all-features
+```
+
+See also: [Clippy usage on CI](.github/workflows/ci.yml)
+
+### Benchmarks
+
+It is always a good idea to run benchmarks before submitting a pull request, especially if there are changes in the
+(de)serialization logic or in the transport layer. Check for more details in
+the [benchmarks overview](./benches/README.md)