[#231] add contributingmd

[ItaloBorrelli]

Fixes #231

Proposed fixes:

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply

[ItaloBorrelli]

See formatted here: https://github.com/cioos-siooc/ckan/blob/231-add-contributingmd/CONTRIBUTING.md

[github-actions]

Image has been pushed to cioos/ckan

Testing Quick Start

Pull image

sudo docker pull cioos/ckan:DEV_PR232
or
sudo CKAN_TAG=DEV_PR232 docker-compose pull ckan

Remove Home Volume and Restart

sudo docker-compose down
sudo docker volume rm docker_ckan_home
sudo CKAN_TAG=DEV_PR232 docker-compose up -d

for full documentation see TBD

[sjbruce]

Looks good, there's a bunch of stuff currently located the other docs, (INSTALL_CIOOS_CKAN.md, FEATURES.md, etc.) that could potentially live in this document or alongside this one but need to be reviewed first.

We can shore these up in subsequent updates & PRs - this a good start.