Add CONTRIBUTING.md#1431

Merged

real-or-random merged 4 commits intobitcoin-core:masterfrom

jonasnick:contributing

Dec 7, 2023

Commits on Dec 4, 2023

Commits on Dec 5, 2023

README: update first sentence
jonasnick
committed

Commits on Dec 6, 2023

CONTRIBUTING: add some coding and style conventions
jonasnick
committed