Clarify contribution guidelines/"awesomeness" criteria

This is a summary of previous discussions (https://github.com/awesome-foss/awesome-sysadmin/issues/460 and others) about what does qualify for inclusion in the list, and what doesn't. This is not definitive, in short, approval is currently purely **at the appreciation of the maintainers**, but the following points are evaluated during reviews:

* Target audience: professional sysadmins (not beginners): **mostly consensus**
* No `curl | bash` as the *only* documented installation method: **consensus**
* Maturity (e.g., old enough to be in Debian Stable): **undecided**
* Usable in a professional setup: **mostly consensus**
* Decently sized community of users or contributors: **undecided**
* OpenSSF Best Practices as a quality indicator: **undecided**
* PR template must be respected, with sufficient answers given: **consensus**
* Be "boring" / extremely conservative and opinionated (not "awesome": rejected): **consensus**
* All listed software must be FOSS: **consensus**
* Remove unmaintained / abandoned projects: **consensus**
* Majority approval (2+ approvals) required to merge PRs: **undecided**
* Self-submissions by project maintainers are undesirable: **consensus**

This summary might be updated after further discussion.

Start by reviewing the summary in this issue and the prior discussion in issue #460, then compare the consensus and undecided criteria. Done means producing an updated, clearly agreed contribution-guidelines document that reflects the maintainers' decisions; the target file is not identified here.

Written by the indexing model from the issue text.