FAQ on installing Fast on a non-empty org

fast/stages/FAQ.md

@@ -11,3 +11,15 @@
 - **How can I fix permission issues when running Terraform apply?**
   - Make sure your account is part of the organization admin group defined in variables.
   - Make sure you have configured [application default credentials](https://cloud.google.com/docs/authentication/application-default-credentials), rerun `gcloud auth login --update-adc` to fix them.
+- **My GCP Org is not empty, what is the best way to save existing work and still install Fast?**
+  - Background: Fast needs to be installed on the org level - because of many things that one can do only on that level, like the org policy role, secure tags, org policies
+  - Create a folder, you can call it "Legacy"
+  - Move all the existing projects and folders into this folder (you can do it by selecting all of them at once on the [resource management page](https://console.cloud.google.com/cloud-resource-manager) of the GCP console)
+  - Collect the existing defined org policies and save them:
+    ```
+    gcloud organizations list
+    export FAST_ORG_ID=123456
+    for c in $(gcloud org-policies list --organization $FAST_ORG_ID --format='get(constraint)'); do gcloud org-policies describe --organization $FAST_ORG_ID $c ; echo '---' ; done > previous_policies.yaml
+    ```
+  - Analyze the policies together with [the ones Fast applies](1-resman/data/org-policies) and apply the ones that still make sense on that "Legacy" folder level
+  - Proceed with installing Fast normally