Update and rename CONTRIBUTING.adoc to CONTRIBUTING.md

Author: LukasBrand

CONTRIBUTING.adoc

@@ -0,0 +1,7 @@
+# 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).