[CI Skip] Create CONTRIBUTING.md

Author: Sefiraat

CONTRIBUTING.md

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+# Contributing
+
+When contributing to this repository, please first discuss the change you wish to make via the Slimefun Addon Community Discord server (https://discord.gg/J4KurMDCKU).
+
+Please note we have a code of conduct, please follow it in all your interactions with the project.
+
+## Pull Request Process
+
+1. Ensure any install or build dependencies are removed before the end of the layer when doing a 
+   build.
+2. Ensure valid annotations and JavaDocs have been added to new, public, methods and code has been commented for clarify where appropriate.
+3. Increase the version numbers in the pon.xml file to the new version that this
+   Pull Request would represent. The versioning scheme we use is [SemVer](http://semver.org/).
+4. You may merge the Pull Request in once you have the sign-off of one other developer, or if you 
+   do not have permission to do that, you may request the reviewer to merge it for you.