Add ADR 002 which partially pivots on ADR 001

docs/ADR/002-pivot-how-we-port-to-postgres.md

@@ -0,0 +1,13 @@
+# ADR 002 - Pivot on how we port to PostgreSQL
+
+## Context
+
+This partially reverses the approach outlined in [ADR 001](001-porting-to-postgresql.md). We were going to piggy back on work by the Publishing Team who are building a proxy app to split requests between old and new router/router-api and content-store. However they've hit several issues and can't get it working.
+
+On recommendation from the Tech Lead of the Platform Engineering team we are going to just use feature flags to manage the roll out.
+
+For feature flagging we will check for the presence of an environment variable. The default behaviour will be to use MongoDB if the variable isn't set or does not have the value "postgres". The variable should be called "ROUTER_USE_DB" and the values "mongo" or "postgres" to be absolutely clear.
+
+## Status
+
+Accepted