Add contributing doc

CONTRIBUTING.md

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+# Contributing
+
+Are you interested in contributing code to Kotter? First of all, let me say -- thanks! I really mean it. I appreciate
+your time and that you're giving some of it to this project.
+
+As it's still very early in the life of the project, the requirements set out for now will be pretty minimal, but I
+expect to tighten them over time as the project stabilizes.
+
+First of all, **be aware of our code of conduct**
+
+* For now, we're borrowing almost wholesale from the
+  [Rust Code of conduct](https://www.rust-lang.org/policies/code-of-conduct)
+* TL;DR - debate is welcome to generate the best ideas, but respect each other. We'll never tolerate behavior where
+  someone is affecting the community negatively or attacking people and not ideas.
+
+Other than that, just a few rules:
+
+* **Code consistency and readability are the highest priorities right now**
+    * Consider using IntelliJ IDEA (Community Edition is fine). This way, we can set common project settings in a file
+      and lean on the tools to resolve disagreements, so we can focus on more interesting work.
+    * Changes should be tested.
+* **Pull requests should be associated with a bug that you have assigned to yourself**
+    * This ensures that we won't have multiple people working on the same issue at the same time
+* **New features should be discussed with me first**
+    * This ensures that you won't waste your time on something I might have reasons to reject.
+    * Of course, pull requests can be an effective way of proposing a feature, so go for it as long as you're OK that
+      there are no guarantees we'll take the change.
+    * Ways to connect:
+        * [Join my Discord!](https://discord.gg/5NZ2GKV5Cs)
+        * Follow me on Twitter: [@bitspittle](https://twitter.com/bitspittle)
+        * You can send direct queries to [my email](mailto:[email protected]).
+
+These rules are NOT dogma. I am always open to exceptions, but be ready to explain why your situation is special!
+
+Thank you, again, for choosing to help out on Kotter.