Set up Architecture Decision Records

[Relequestual]

We had a Discussion to see if we should adopt Architecture Decision Records (ADRs). There were no strong objections and a few clear benefits.

We should add ADRs to this repo, the spec repo, and the test repo.
I suggested we use ISO date as the file prefix as opposed to incrementing numbers, as this is less likely to cause a conflicts. This approach was also adopted for proposals in OAI.

Work to be done:

• Set up template as per https://adr.github.io/madr for this repo - (Add first Architecture Decision Record! #21 )
• Add ADR badge to this repo - (59adf96)
• Update CONTRIBUTING.md in this repo - (Add contributing guide, mainly focusing on how to use ADRs #33 )
• Create minimal Issue and PR templates in this repo (Add Issue and Pull Request templates, mainly for ADRs #36)
• Add the same to all repos*

* I'm not currently clear if "all repos" is appropriate. For example, it wouldn't make sense for the mostly empty vocabularies repo. I think it makes sense to put these in a .github repo for the org, but I'm not sure they are currently working as expected.

[Relequestual]

I'm tempted to create a new issue for "roll out to all repos".

In the first instance, it makes sense to just add the Issue and PR templates to a .github repo.
Later we can consider automating distribution of templates and other community files to specific repos.

@derberg has developed a tool for globally passing out workflows within a GitHub org, but we might want a modified version which can also handle other files.