Clarify process for becoming core committers

[mbernasocchi]

Initial proposal by Marco

We should fix a rule where the PSC has not much to say in this and to successfully become a new core committer a contributor needs to be vouched for by at least 5 (it might be
3, 5, 5 with at least 3 non-coworkers, X) core committers and no major concerns have risen?
The PSC would then just acknowledges what the core developer community decided and if all is
good, award the core-committer status to the nominee.

I want to be very welcoming and encourage new committers to aspire to
become core committer, but I'd still like the core committer "title" to
be given out only when the developer community thinks it is really deserved.

Notes

https://lists.osgeo.org/pipermail/qgis-psc/2020-May/008763.html

[anitagraser]

For reference: we used Loomio for voting for a while https://www.loomio.org/g/EKV14L8A/qgis

[anitagraser]

ongoing at https://docs.google.com/document/d/1p7JGK-t7P7lrXPIxwjhc3mSo0TaOTfUaIypFzO3i17I/edit