Add first Architecture Decision Record!

Resolves first part of #20

docs/adr/2021-05-17-use-markdown-architectural-decision-records.md

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+# Use Markdown Architectural Decision Records
+
+* Status: accepted
+* Deciders: @gregsdennis, @Julian, @jdesrosiers, @karenetheridge
+* Date:  2021-06-17
+
+## Context and Problem Statement
+
+We want to record architectural decisions made in this project.
+Which format and structure should these records follow?
+
+## Considered Options
+
+* [MADR](https://adr.github.io/madr/) 2.1.2 – The Markdown Architectural Decision Records
+* [Michael Nygard's template](http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions) – The first incarnation of the term "ADR"
+* [Sustainable Architectural Decisions](https://www.infoq.com/articles/sustainable-architectural-design-decisions) – The Y-Statements
+* Log4Brains <https://github.com/thomvaill/log4brains>
+* Other templates listed at <https://github.com/joelparkerhenderson/architecture_decision_record>
+* Formless – No conventions for file format and structure
+
+## Decision Outcome
+
+Chosen option: "MADR 2.1.2", because
+
+* Implicit assumptions should be made explicit.
+  Design documentation is important to enable people understanding the decisions later on.
+  See also [A rational design process: How and why to fake it](https://doi.org/10.1109/TSE.1986.6312940).
+* The MADR format is lean and fits our development style.
+* The MADR structure is comprehensible and facilitates usage & maintenance.
+* Version 2.1.2 is the latest one available when starting to document ADRs.
+
+We agreed to not require the use of any specific tooling.
+Using MADR can be done manually by manually copying the template and manually updating the index.md file. Doing both of these manually is arguably easier, and there are not many well maintained tools.
+
+## Links
+
+* Proposal: [GitHub Discussion - We should adopt Architecture Decision Records #15 ](https://github.com/json-schema-org/community/discussions/15)
+* Issue: [Set up Architecture Decision Records #20](https://github.com/json-schema-org/community/issues/20)

docs/adr/index.md

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+# Architectural Decision Log
+
+This log lists the architectural decisions for JSON Schema Community.
+
+<!-- adrlog -- Regenerate the content by using "adr-log -i". You can install it via "npm install -g adr-log" -->
+
+* [ADR-2021-05-17](2021-05-17-use-markdown-architectural-decision-records.md) - Use Markdown Architectural Decision Records
+
+<!-- adrlogstop -->
+
+For new ADRs, please use [template.md](template.md) as basis.
+More information on MADR is available at <https://adr.github.io/madr/>.
+General information about architectural decision records is available at <https://adr.github.io/>.

docs/adr/template.md

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+# [short title of solved problem and solution]
+
+* Status: [proposed | rejected | accepted | deprecated | … | superseded by [ADR-0005](0005-example.md)] <!-- optional -->
+* Deciders: [list everyone involved in the decision] <!-- optional -->
+* Date: [YYYY-MM-DD when the decision was last updated] <!-- optional -->
+
+Technical Story: [description | ticket/issue URL] <!-- optional -->
+
+## Context and Problem Statement
+
+[Describe the context and problem statement, e.g., in free form using two to three sentences. You may want to articulate the problem in form of a question.]
+
+## Decision Drivers <!-- optional -->
+
+* [driver 1, e.g., a force, facing concern, …]
+* [driver 2, e.g., a force, facing concern, …]
+* … <!-- numbers of drivers can vary -->
+
+## Considered Options
+
+* [option 1]
+* [option 2]
+* [option 3]
+* … <!-- numbers of options can vary -->
+
+## Decision Outcome
+
+Chosen option: "[option 1]", because [justification. e.g., only option, which meets k.o. criterion decision driver | which resolves force force | … | comes out best (see below)].
+
+### Positive Consequences <!-- optional -->
+
+* [e.g., improvement of quality attribute satisfaction, follow-up decisions required, …]
+* …
+
+### Negative Consequences <!-- optional -->
+
+* [e.g., compromising quality attribute, follow-up decisions required, …]
+* …
+
+## Pros and Cons of the Options <!-- optional -->
+
+### [option 1]
+
+[example | description | pointer to more information | …] <!-- optional -->
+
+* Good, because [argument a]
+* Good, because [argument b]
+* Bad, because [argument c]
+* … <!-- numbers of pros and cons can vary -->
+
+### [option 2]
+
+[example | description | pointer to more information | …] <!-- optional -->
+
+* Good, because [argument a]
+* Good, because [argument b]
+* Bad, because [argument c]
+* … <!-- numbers of pros and cons can vary -->
+
+### [option 3]
+
+[example | description | pointer to more information | …] <!-- optional -->
+
+* Good, because [argument a]
+* Good, because [argument b]
+* Bad, because [argument c]
+* … <!-- numbers of pros and cons can vary -->
+
+## Links <!-- optional -->
+
+* [Link type] [Link to ADR] <!-- example: Refined by [ADR-0005](0005-example.md) -->
+* … <!-- numbers of links can vary -->