Add upgrade instructions

website/site/content/docs/install/upgrading.md

@@ -0,0 +1,35 @@
++++
+title = "Upgrading"
+weight = 35
++++
+
+Upgrading docspell requires to download the newer binaries and
+re-installing them depending on your setup.
+
+The database is migrated on application start automatically.
+
+Since [downgrading](@/docs/install/downgrading.md) is not supported,
+it is recommended to backup your database before upgrading. Should
+something not work as expected, restore the database backup and go
+back to the previous version.
+
+# Docker-Compose
+
+When using the provided `docker-compose` setup, you only need to pull
+the new images. The latest release is always tagged with `-LATEST`.
+
+``` bash
+$ docker-compose down
+$ docker-compose pull
+$ docker-compose up --force-recreate --build -d
+```
+
+# Zip / Deb Files
+
+When using the zip or deb files, either install the new deb files via
+your package manager or download and unpack the new zip files.
+
+# Nix
+
+When using the provided nix setup, the `currentPkg` always points to
+the latest release. Thus it is enough to run `nix-build`.