upgrade docs

docs/UPGRADE.md

@@ -1,9 +1,22 @@
-# Upgrade from earlier versions
+# Upgrading old versions
 
-I very much recommend going via each major release in turn (eg, 2.0.6
+## Get the new code (from git)
--> 2.1.3 -> 2.2.4 -> 2.3.0 rather than 2.0.6 -> 2.3.0).
 
-While the basic database and file formats haven't changed *completely*, it's
+If you got shimmie from `git`, then `git pull` should fetch the latest code
-different enough to be a pain.
+(`git checkout master` then `git pull` if you're on an old non-master branch).
+
+Once the new Shimmie code is ready, you'll need to make sure all the
+dependencies are in place and up-to-date via `composer install`.
 
 
+## Get the new code (.zip)
+
+If you got shimmie from one of the .zip downloads, you'll need to download
+new code, extract it, then copy across the `data` folder from the old install
+into the new one.
+
+
+# Update database schema
+
+This should be automatic - next time the site is loaded, it'll see that the
+current schema is out of date, and will update it.