Add contributing guidelines

CONTRIBUTING.md

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+# Contributing
+
+## Cloning for Contribution
+To set up the development environment to make changes to the code, clone the repo, install dependencies, and run it directly with `node` as Docker doesn't expose as much.
+
+## Making Changes
+To edit the code please keep a few things in mind:
+1. Be able to explain changes you make; they should be legible and understandable.
+2. Please try to abide by ESLint's rules, setting up ESLint is quite easy to do and not too difficult to follow.
+
+## Testing Your Code
+For testing, make sure to set up the prerequisites:
+1. MongoDB: have a MongoDB server set up.
+2. Environment Variables: take a look at the `docker-compose.yml` for examples.
+
+Once that's done, you can run `yarn tsc` or `npx tsc` to build to JavaScript in the `built` directory. Here you'll find `deploy-commands.js` which you need to deploy the slash commands, and `index.js` which you can run to start the bot.
+
+## Submitting Your Contribution
+Creating a PR is easy enough and there are plenty of tutorials to do so. When contributing, please make sure to highlight and explain your change, with any import information additionally included.
+
+---
+
+# Thank you for any contributions!
+
+---