Update contribution description

Update the CONTRIBUTION.md to conform with our current status and CLA
process

Author: smcvb

CONTRIBUTING.md

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+# Contribution Guidelines
+
+Thank you for your interest in contributing to the Axon Framework Kotlin Extension. To make sure using Axon is a smooth
+experience for everybody, we've set up a number of guidelines to follow.
+
+There are different ways in which you can contribute to the framework:
+
+1. You can report any bugs, feature requests or ideas about improvements on
+   our [issue page](https://github.com/AxonFramework/extension-kotlin/issues/new/choose). All ideas are welcome. Please
+   be as exact as possible when reporting bugs. This will help us reproduce and thus solve the problem faster.
+2. If you have created a component for your own application that you think might be useful to include in the framework,
+   send us a pull request (or a patch / zip containing the source code). We will evaluate it and try to fit it in the
+   framework. Please make sure code is properly documented using JavaDoc. This helps us to understand what is going on.
+3. If you know of any other way you think you can help us, do not hesitate to send a message to
+   the [AxonIQ's discussion platform](https://discuss.axoniq.io/).
+
+## Code Contributions
+
+If you're contributing code, please take care of the following:
+
+### Contributor Licence Agreement
+
+To keep everyone out of trouble (both you and us), we require that all contributors (digitally) sign a Contributor
+License Agreement. Basically, the agreement says that we may freely use the code you contribute to the Axon Framework
+Kotlin Extension, and that we won't hold you liable for any unfortunate side effects that the code may cause.
+
+To sign the CLA, visit: https://cla-assistant.io/AxonFramework/extension-kotlin
+
+### Code Style
+
+We're trying very hard to maintain a consistent style of coding throughout the code base. Think of things like indenting
+using 4 spaces, putting opening brackets (the '{') on the same line and putting proper JavaDoc on all non-private
+members.
+
+If you're using IntelliJ IDEA, you can download the code style
+definition [here](https://github.com/AxonFramework/AxonFramework/blob/master/axon_code_style.xml). Simply import the XML
+file in under "Settings -> Code Style -> Scheme -> Import Scheme". Doing so should make the code style selectable
+immediately.