Add retrospective ADR for usage of remote settings

[mythmon]

This may help when reviewing recent problems with the signing infrastructure, and to better undrest and Normandy's history.

Fixes #2309

[mythmon]

bors try

[bors[bot]]

try

Build succeeded:

• ci/circleci: lint
• ci/circleci: docs-build
• ci/circleci: python-tests
• ci/circleci: docker-image-build-pr

[mythmon]

bors r= @rehandalal

[bors[bot]]

Build succeeded:

• ci/circleci: docker-image-build-pr
• ci/circleci: docs-build
• ci/circleci: lint
• ci/circleci: python-tests