Add a section on what a client should have to be approved to CONTRIBUTING.md

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Describe your suggestion

Because, for example, discord-aos which can't even load the messages list got approved. And my client which can be installed without going into terminal or installing a bunch of packages, already has a lot of features and being actively developed got rejected.

Please add a section about this so we can understand what a client which is not "early" or "WIP" means.

Acknowledgements

• [X] I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open issue.
• [X] I have written a short but informative title.
• [ ] I have admitted that I am a clown by having checked this box, as I have not read these acknowledgements.
• [X] I will fill out all of the requested information in this form.

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Woah, sorry, I thought a contributor is a user with repository access. Closing