Merge pull request #266 from mozilla-it/adr-1

Adding doc/diagram around fxa to ctms integration

adrs/madr-fxa-ctms.md

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+# Stripe Subscription Event Data Empowering Email User Journeys
+
+* Status: proposed
+* Deciders: Bryan Sieber, John Whitlock, Ben Bangert, Jon Buckley, (Benson Wong, Stephen Hood)
+* Date: September 30, 2021
+
+Technical Story: [design and document approach for marketing | https://mozilla-hub.atlassian.net/jira/software/c/projects/CTMS/boards/364?modal=detail&selectedIssue=CTMS-73]
+
+## Context and Problem Statement
+
+Marketing wants to be able to differentiate customers based on their subscription information to empower marketing campaigns.
+
+## Decision Drivers
+
+* Design sustainable system for longterm data-health
+* FxA source of Truth
+* Stripe rate limits
+
+
+## Considered Options
+
+* Cloud Function triggered through Firestore
+* Background process in CTMS
+* New temporary service
+
+## Decision Outcome
+
+Chosen option: " Cloud Function triggered through Firestore", because the event snapshots can be captured and published to a Pub/Sub, upon which they can be pushed to CTMS with a JWT. We believe this is a more sustainable option.