Add a Contributing guide file

[NotHyper-474]

In the Contributing section, clicking the hyperlink will attempt to open a missing 'CONTRIBUTORS.MD' file

Please make one, it could, for instance, explain that you should make code contributions (via pull requests) basing on the main branch if it's a documentation change and develop if it's a code change.

Here's more or less how I think this file would go:

Thanks for the initiative to contribute to Friday Night Funkin'! You can help by either:
* Creating an issue report if you spot a bug or experience a crash, but make sure to see if that was not already reported before to not make duplicates!
* Making a pull request with either documentation changes or code contributions! Please, as with issue reports, search if another pull request didn't do that already! Read more details on how to contribute with code contributions below.

Footnote: Please make sure you're also on the right repository, if you want to report something for an engine, do it on their repository instead!

### Contributing with code or documentation 
If you're making changes to documentation, which includes any file that ends with `.md`, you should base your changes on the `main` branch.  
Otherwise, you should base it on the `develop` branch.

If you're doing changes on either the `assets` or `art` submodules, please make the pull request over their respective repositories.

[Hundrec]

If Eric and Mr. Cameron Muffin have a rough idea of what should be in the Contributor's Guide, please let us know!