Dev helpers

[cmdoret]

This PR provides utilities to make development easier:

• Contributing section in README.
• Makefile to automate common recipes.
• Dockerfile to build a “ready-to-use” image (i.e., Bioconductor + HiCExperiment + Companion packages).
• GitHub actions to automatically publish a docker image on ghcr.io/js2264/hicexperiment.
  • Triggered whenever there is a new [GitHub] release
  • Image tag is taken from the Version field in DESCRIPTION
• File for unit tests are now automatically downloaded on devtools::test(), without having to source testthat.R

Note: The GitHub actions may require enabling the container registry for the repo and making the "package" public.

[js2264]

Note that on top of this, run_docker is enabled in rworkflows and is compiling an image sent to Docker Hub on every push. I clarified it in 7f73f79.