Added contribution guidelines

Since there were no set out guidelines for contributing, I decided to add them.

The guidelines overall are very straightforward.

CONTRIBUTING.md

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+# How to contribute to the FlaxEngine
+
+For any questions, suggestions or help join our discord!
+
+<a href="https://flaxengine.com/discord"><img src="https://discordapp.com/api/guilds/437989205315158016/widget.png"/></a>
+
+Want to see whats planned for Flax?
+
+Go check out our [Trello](https://trello.com/b/NQjLXRCP/flax-roadmap).
+
+## **Found a bug?**
+
+* Avoid opening any new issues without having checked if your problem has already been reported. If there are no currently open issues that fit your problem's description, feel free to [add it](https://github.com/FlaxEngine/FlaxEngine/issues/new).
+
+* When writing an issue make sure to include a clear title and description as well as having filled out all the necessary information, depending on the severity of the issue also include the necessary log files and minidump.
+
+* Try to following the given template when writing a new issue if possible.
+
+## **Want to contribute?**
+  
+* When creating a PR for fixing an issue/bug make sure to describe as to what led to the fix as well as mentioning the
+  relevant issue where it was first mentioned if necessary, for small and obvious fixes this is not needed.
+  
+* For feature PR's the first thing you should evaluate is the value of your contribution, as in, would what it bring to this engine? 
+  If its a small change you could preferably suggest it to us on our discord, else feel free to open up a PR for it.
+  
+* Ensure when creating a PR that your contribution is well explained with a adequate description and title.
+
+* For large contributions, generally good code quality is expected, make sure your contribution works as intended and is appropriately commented where necessary.
+
+
+Thank you for taking interest in contributing to Flax!