Merge pull request #515 from hivemq/improvement/update-contributing

Update CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Welcome to the HiveMQ Community!
+Glad to see your interest in contributing to HiveMQ Community Edition.
+Please checkout our [Contribution Guide](https://github.com/hivemq/hivemq-community/blob/master/CONTRIBUTING.adoc) to make sure your contribution will be accepted by the HiveMQ team.
+
+For information on how the HiveMQ Community is organized and how contributions will be accepted please have a look at our [HiveMQ Community Repo](https://github.com/hivemq/hivemq-community).
+
+## Checking out the Project
+
+We recommend that you use the [IntelliJ](https://www.jetbrains.com/idea/download/) IDE for all development on HiveMQ CE.
+It will automate your process as much as possible.
+Java version 11 is required to build and run HiveMQ CE (for example [Azul Zulu JDK](https://www.azul.com/downloads/)).
+You can check your installed Java version by entering `java -version` on the command line.
+
+First you need to [fork](https://help.github.com/en/articles/fork-a-repo) the HiveMQ Community Edition [repository](https://github.com/hivemq/hivemq-community-edition).
+
+Then you can clone the repository:
+
+```shell
+git clone https://github.com/<your user name>/hivemq-community-edition.git
+```
+
+*Open* the HiveMQ CE project folder in IntelliJ.
+Choose to sync the gradle project, if so prompted by the IDE.
+After setting the gradle and project SDK (Java 11), you are good to go.
+
+### Checking out the HiveMQ Extension SDK
+
+HiveMQ CE uses the HiveMQ Extension SDK which resides in [its own repository](https://github.com/hivemq/hivemq-extension-sdk).
+By default, you can not use the latest changes of or modify the Extension SDK.
+Please checkout the `hivemq-extension-sdk` repository next to the `hivemq-community-edition` repository.
+Gradle and IntelliJ will then automatically wire the two projects.
+
+If you only want to use the Extension SDK, execute the following command from the `hivemq-community-edition` project directory:
+
+```shell
+git clone https://github.com/hivemq/hivemq-extension-sdk.git ../hivemq-extension-sdk
+```
+
+If you also want to make changes to the Extension SDK, please fork the `hivemq-extension-sdk` repository and clone your fork:
+
+```shell
+git clone https://github.com/<your user name>/hivemq-extension-sdk.git ../hivemq-extension-sdk
+```
+
+## 🚀 Thank you for taking the time to contribute to HiveMQ Community Edition! 🚀
+
+We truly appreciate and value your time and work. ❤️