[Documentation] Add upgrade procedure from community to enterprise (#1596)

Co-authored-by: Adam Janikowski <12255597+ajanikow@users.noreply.github.com>

Author: kvahed

docs/upgrading.md

@@ -7,10 +7,10 @@ one version to the next.
 It is highly recommended to take a backup of your data before upgrading ArangoDB
 using [arangodump](https://docs.arangodb.com/stable/components/tools/arangodump/) or [ArangoBackup CR](backup-resource.md).
 
-## Upgrade an ArangoDB deployment
+## Upgrade an ArangoDB deployment version
 
 To upgrade a cluster, change the version by changing
-the `spec.image` setting and the apply the updated
+the `spec.image` setting and then apply the updated
 custom resource using:
 
 ```bash
@@ -29,3 +29,25 @@ is stopped, then the new version is started with `--database.auto-upgrade`
 and once that is finish the new version is started with the normal arguments.
 
 The process for major level upgrades depends on the specific version.
+
+## Upgrade an ArangoDB deployment to Enterprise edition
+
+In order to upgrade a cluster from community to enterprise, we have to
+go through to the following adjustements to an existing deployment:
+
+* [Add a license key](https://arangodb.github.io/kube-arangodb/docs/how-to/set_license.html)
+to the cluster
+* Adjust `spec.image` to a valid enterprise image string
+* Add `spec.license.secretName` to the introduced license key
+
+```bash
+kubectl apply -f yourCustomResourceFile.yaml
+```
+
+The actual upgrade procedure follows exactly the same steps as
+described above for a simple version upgrade in a sequential manner.
+
+Regardless of if you are not only changing the images of community and
+enterprise of the same major, minor and patch levels, or upgrade both
+to enterprise and a different version, the procedure is only performed
+once in a combined step of upgrading version and edition.