Add contributing guide

[cirocosta]

Feature Request

What challenge are you facing?

There are certain steps that we need to go through before submitting a PR to this repository, e.g., ensuring that the erb templates were properly generated.

Currently, that's "tribal knowledge" spread across the team that is not openly communicated (and sometimes, forgotten by us 😅 )

A Modest Proposal

Have a CONTRIBUTING.md file, just like we do in concourse/concourse (see contributing.md).

[cirocosta]

Oh, I just realized that @taylorsilva just had #65 in 🤔

[taylorsilva]

This is a great idea if there are other things to capture. Running that script for the job templates is only one thing, but we also have the entire /dev directory that could go into a contributing.md, maybe?

What other items could go into a contributing.md?

[cirocosta]

yeah, perhaps documenting the dev stuff would be good indeed, e.g., how we document how to build and run Concourse in the main contributinig one:

After making any changes, you can try them out by rebuilding and recreating the web and worker containers:
$ docker-compose up --build -d
[...]

(see https://github.com/concourse/concourse/blob/master/CONTRIBUTING.md#rebuilding-to-test-your-changes)

[cirocosta]

oh, something else:

• Add CONCOURSE_GARDEN_REQUEST_TIMEOUT config #66 (review)

giving some guidance on adding things like example:, descriptions, not setting defaults, etc would be pretty nice