Add a guide for Concourse CI/CD

[joeduffy]

I've gotten asked about integrating with Concourse a number of times over the past couple weeks. Seems like a good thing to have on our roadmap of CI/CD integrations.

[lukehoban]

See also #598.

[ringods]

@joeduffy I'm busy working on a Pulumi resource type for Concourse.

https://github.com/ringods/pulumi-resource

Once it is working, I will see if I can find the time to contribute some documentation for the Pulumi website.

[desteves]

Do we still want to add this to our roadmap?

I had a quick chat with @ringods last week, and we agree Concourse is less relevant nowadays.

[joeduffy]

I think we can keep this open in case others want to comment (there are six upvotes), however, it's definitely not near the top of the priority list. I'd be curious to hear feedback from the community -- and, as always, pull requests are welcome!