Add ADR for porting router and router-api to PostgreSQL

docs/ADR/001-porting-to-postgresql.md

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+# ADR 001 - How we migrate Router/Router-API from Mongo to Postgres
+
+## Context
+
+We need to migrate router (and [router-api](https://github.com/alphagov/router-api)) off of MongoDB as its datastore. The majority of GOV.UK platform is on PosgreSQL and there is no longer a strong enough case for being on MongoDB for router data.
+
+Even the [content-store is migrating from MongoDB to PostgreSQL](https://github.com/alphagov/govuk-rfcs/pull/158).
+
+This ADR is to outline our decisions and rationale on the steps we take for that migration:
+
+### The outline
+* Spin up a completely separate stack of router and router-api (govuk-docker first) that will talk to a PostgreSQL db only (as forks)
+* Provision in EKS a new PostgreSQL db
+* Provision the new p-router/p-router-api apps
+* Amend messages to router-api to be duplicated to p-router-api within content-store
+
+### Benefits of doing it this way
+* As updates are being made to the two databases, we can backfill data previous to that point in time without loss of service or downtime to the live (old) service
+* We can test traffic going to the new p-router stack insolation
+* Switchover would mean a change in DNS
+* We can rename p-router once we decommission old router
+
+### Risks
+
+* We never rename p-router
+* DNS messes up and caching gets in the way, causing downtime to requests trying to reach origin
+
+## Alternatives
+
+* We update router and router-api and deploy the changes to the existing version
+
+### Risks
+
+* Big bang
+* Running 2 databases under 1 app
+* Double code/logic to handle that
+* Harder to clean up
+
+## Decision
+
+* https://trello.com/c/nQcpWnD5/1788-provision-new-apps-in-govuk-docker-for-prouter-and-prouter-api
+* https://trello.com/c/dOXADTQP/1802-spin-up-a-separate-stack-of-router-api-l
+* https://trello.com/c/YJro3BGj/1790-spin-up-a-separate-stack-of-router-l
+
+## Status
+
+Accepted