Consider ADRs

[tweag-ev-ak]

This is a suggestion to adopt ADRs as a formal structure for the implementation choices document https://github.com/tweag/cardano-canonical-ledger/blob/main/docs/implementation-choices.md.

It became wide-spread among cardano ecosystem packages:

• https://github.com/cardano-scaling/hydra/blob/master/docs/adr/2021-06-05_001-record-architectural-decisions.md
• https://github.com/IntersectMBO/plutus/tree/master/doc/adr
• https://github.com/IntersectMBO/cardano-ledger/tree/master/docs/adr