Add general, org-wise contribution (and maintainance) guidelines

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1. How to find where/how to contribute
2. How to file an issue/send a PR
3. Brief maintainers guide

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Worth mentioning: maintainers may push directly to the feature branch to fix minor issues

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Update https://github.com/gbdev/meta/blob/master/CONTRIBUTING.md and link to it

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done here: https://gbdev.io/contributing.html