Begin recording architecture decisions

[ahouseholder]

This PR adds the /docs/adr directory and creates two starter architecture decisions

• 0000 to record architecture decisions
• 0001 to use MADR format for them

References:

• https://adr.github.io/
• https://adr.github.io/madr/

Rationale:

• We need a place to capture the outcomes of decisions within the project.
• ADR (and the MADR format in particular) seem to be a good enough place to start.

I can see this being where we capture things like

• why some things get versioned and others do not
• why SSVC decision points are ordered sets
• why SSVC outcome sets are ordered sets
• why we treat trees as examples not strict rules
• and even some of the things that went into the original paper (like the decision trees vs other alternatives)

[j---]

This is a good idea, thank you.