CONTRIBUTING.md: Clarify guidelines for `meta.description`

Changed files
+1
+1
CONTRIBUTING.md
···
* Not start with the package name.
* More generally, it should not refer to the package name.
* Not end with a period (or any punctuation for that matter).
+
* Aim to inform while avoiding subjective language.
* `meta.license` must be set and fit the upstream license.
* If there is no upstream license, `meta.license` should default to `lib.licenses.unfree`.
* If in doubt, try to contact the upstream developers for clarification.