Set up CONTRIBUTING.md

- moved into place from doc/dev/contrib.md
- build CONTRIBUTING from CONTRIBUTING.md, like README and OPTIONS
4 jobs for 96-update-contributors-guide-and-move-it-at-top-level-so-it-s-recognised-by-gitlab in 11 minutes and 7 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Build
passed #2615
linux docker
build:debian:sid:amd64

00:05:03

passed #2616
linux docker
build:debian:sid:i386

00:05:19

 
  Test
passed #2617
linux docker
test:debian:sid:amd64

00:05:47

passed #2618
linux docker
test:debian:sid:i386

00:05:46