docs: Update README with details of how to deploy and update

README.md

@@ -1,2 +1,12 @@
 # freeagent-ssosync
-Runs https://github.com/awslabs/ssosync in a container for lambda usage
+Runs https://github.com/awslabs/ssosync in a container for lambda usage.
+
+## Deployment
+
+Merging will deploy a `latest` tag image which will get used automatically.
+
+## Development
+
+This pulls in SSOSync from the upstream git repository, based on the version argument in `Dockerfile`.
+
+Note, there is a fork at [fac/ssosync](https://github.com/fac/ssosync) we could use if we need to customise and/or test features but we should rely on the upstream for production unless absolutely necessary.