pkgs/README: Clarify library dependent incluson criterion (#425473)

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pkgs
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pkgs/README.md
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* How realistic is it that it will be used by other people?
It's good that nixpkgs caters to various niches, but if it's a niche of 5 people it's probably too small.
A good estimate is checking upstream issues and pull requests, or other software repositories.
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Library packages should have at least one dependent.
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* Library packages should have at least one dependent.
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If possible, that dependent should be packaged in the same PR the library is added in, as a sanity check.
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If it is not possible to package the dependent, a minimal test program should be added to `passthru.tests`.
* Is the software actively maintained upstream?
Especially packages that are security-critical, rely on fast-moving dependencies, or affect data integrity should see regular maintenance.
* Are you willing to maintain the package?