add README, add ci, update opam

1 job for prepare-release in 2 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #366643052
gitlab-org
build

00:02:01

 
Name Stage Failure
failed
build Build
Using Docker executor with image ocaml/opam2:4.07 ...
Pulling docker image ocaml/opam2:4.07 ...
Using docker image sha256:08e0da11442873dfc97605599df9477f4ce81e57fdf8d835cc111d6c1b83b310 for ocaml/opam2:4.07 ...
Running on runner-9538b0ab-project-15631694-concurrent-0 via runner-9538b0ab-gsrm-1575292224-1f03c126...
Fetching changes with git depth set to 50...
Initialized empty Git repository in /builds/dune-network/dune-metal/.git/
Created fresh repository.
fatal: couldn't find remote ref refs/heads/prepare-release
ERROR: Job failed: exit code 1