Add a installer readme

This file is to clarify why this folder is special,
what it does and why it is there.

installer/README.md

@@ -0,0 +1,27 @@
+# Installer
+
+This subfolder contains all resources, which persist across a Cloudron update.
+Only code and assets, which are part of the updater belong here.
+
+Installer is the name which got inherited from times, where this folder contained
+much more infrastructure components, like a local webserver to facilitate updates.
+
+
+## installer.sh
+
+The main entry point for initial provisioning and also updates (not upgrades).
+
+It is called from:
+ * cloudron-setup (during initial provisioning, restoring or upgrade)
+ * cloudron.js in the box code (during an update)
+
+Two arguments need to be supplied in this order:
+ 1. The public url to download the box release tarball `--sourcetarballurl`
+ 2. JSON object which contains the user-data `--data`
+
+
+## box-setup.sh
+
+This is the systemd unit file script hook, which persists Cloudron updates.
+Mostly it revolves around setting up various parts of the filesystem, like btrfs
+volumes and swap files