Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Oat++
+
+Thank you for your interest in the project, and taking time to contribute.  
+
+## How to contribute
+
+All kind of contributions are very welcome.
+
+### Join the community
+
+- Join discussion on **Gitter**. [oat++ framework/Lobby](https://gitter.im/oatpp-framework/Lobby)
+- Follow us on **Twitter** for latest news. [@oatpp_io](https://twitter.com/oatpp_io)
+- Join community on **Redit**. [r/oatpp](https://www.reddit.com/r/oatpp/)
+
+### Try to use Oat++
+
+- Grab example project from the [list of examples](https://github.com/oatpp/oatpp-examples)
+- Take starter project and try to build something on top of it
+  - [Starter project with Simple API](https://github.com/oatpp/oatpp-starter)
+  - [Starter project with Async API](https://github.com/oatpp/oatpp-starter-async)
+- Try to reproduce benchmark results [2M WebSockets](https://github.com/oatpp/benchmark-websocket)
+
+### Open an issue
+
+Ask question or report a bug [here](https://github.com/oatpp/oatpp/issues)
+
+### Create a pull request
+
+All kind of [pull requests](https://github.com/oatpp/oatpp/pulls) are very welcome.
+Even if it's just fixing typos, or adding documentation.
+
+*Please make sure your pull requests confirming to overall project code-style.*  
+*Please make sure to notify the community if you are working on an issue so we don't duplicate work!*
+
+### Create Oat++ extension module
+
+Use [oatpp-starter-module](https://github.com/oatpp/oatpp-starter-module) to create oatpp extensions.
+
+## Code of Conduct
+
+Please follow [Code of Conduct](CODE_OF_CONDUCT.md) at all times.  
+Please report unacceptable behavior to oatpp.io@gmail.com.