Added CONTRIBUTING.md (leaderboardsgg#9)

* Added CONTRIBUTING.md

* PR and branch guidance, discord link footnote

* fix link reference syntax

CONTRIBUTING.md

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+# Contributing to leaderboard-backend
+We appreciate your help!
+
+## Before filing an issue
+If you are unsure whether you have found a bug, please consider asking in [our discord][discord] first.
+
+Similarly, if you have a question about a potential feature, [the discord][discord] can be a fantastic resource for first comments.
+
+## Filing issues
+Filing issues is as simple as going to [the issue tracker](https://github.com/speedrun-website/leaderboard-backend/issues), and adding an issue using one of the below templates.
+
+### Feature Request / Task
+```
+{Feature Request/Task}: {short description}
+---
+{Detailed description}
+
+### Affected Functionality
+{Any known functionality impacted, or Unknown if further research needs to be done}
+
+### Other Relevant Issues
+{Links to Relevant Issues}
+```
+
+### Bugs
+```
+Bug: {short description}
+---
+{Summary of bug}
+
+### Step(s) to Reproduce
+{Numbered list of step(s)}
+
+### Expected Result
+{Summary of expected result}
+
+### Actual Outcome
+{Description of actual outcome}
+```
+
+## Contributing code
+### Example code contribution flow
+1. Make a fork of this repo.
+1. Name a branch on your fork something descriptive for this change (eg. `UpdateReadme`). If possible, reference the issue or task you are working on but this is not necessary.
+1. Make your changes.
+1. Include unit tests for all changes, and integration tests if it is appropriate (i.e. new endpoints are added).
+1. Verify your changes pass all existing and new tests. 
+1. Commit your changes (Tip! Please read our [Style guide](#style-guide) to help the pull request process go smoothly).
+1. Push your branch.
+1. Open a pull request to `leaderboardsgg/leaderboard-backend-poc`. If working on a particular issue, please mention that issue in the pull request and describe your changes.
+1. Get your pull request approved.
+1. Get someone to click `Squash and merge`.
+1. [Celebrate][discord] your amazing changes! 🎉
+
+## Style guide
+### General
+- Be inclusive, this is a project for everyone.
+- Be descriptive, it can be hard to understand abbreviations or short-hand.
+
+### C#
+- Add tests for any new feature or bug fix, to ensure things continue to work.
+- Early returns are great, they help reduce nesting!
+
+### Git
+- Do not make pull requests from `main`.
+- Do not include slashes in your branch name.
+	- Nested paths can act strange when other people start looking at your branch.
+- Try to keep commit line length below 80 characters. If more information is needed, include it in the commit body.
+- Commits should be as [atomic](https://www.freshconsulting.com/insights/blog/atomic-commits/) as possible.
+
+[discord]:https://discord.gg/TZvfau25Vb