docs: Add upgrade guide

docs/setup/upgrading.md

@@ -0,0 +1,26 @@
+---
+title: "Upgrading your Snikket server"
+date: 2021-05-19T14:32:02Z
+---
+
+Upgrading to a new Snikket release is typically very easy.
+
+## snikket-selfhosted
+
+If you installed Snikket using the [snikket-selfhosted][] scripts, simply run:
+
+    cd /opt/snikket
+    git pull
+    ./scripts/update.sh
+
+## Snikket quickstart
+
+If you're using a version installed from the [original quickstart][] guide on
+the website, use these commands instead:
+
+    cd /etc/snikket
+    docker-compose pull
+    docker-compose up -d
+
+[snikket-selfhosted]: https://github.com/snikket-im/snikket-selfhosted
+[original quickstart]: https://snikket.org/service/quickstart/