Docker: Add README.txt to new cloud-init example config

docker/examples/cloud-init/README.txt

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+PhotoPrism for Cloud Servers running Ubuntu 20.04 LTS (Focal Fossa)
+===================================================================
+
+Attention: This config example is under development, and not stable yet!
+
+Run this script to install PhotoPrism on a cloud server e.g. at DigitalOcean:
+
+  bash <(curl -s https://dl.photoprism.org/docker/cloud-init/setup.sh)
+
+This may take a while to complete. Then open
+
+  http://<YOUR SERVER IP>:2342/
+
+in a Web browser to log in using the initial admin password "insecure".
+Please immediately change it in Settings to protect your new PhotoPrism
+installation.
+
+All files related to PhotoPrism can be found in /photoprism. The server
+will be running as "photoprism" (UID 1000). There should be no need to
+change defaults unless other services are running on the same server
+and there are conflicts.
+
+## System Requirements ##
+
+We recommend hosting PhotoPrism on a server with at least 2 cores and
+4 GB of memory. Beyond these minimum requirements, the amount of RAM
+should match the number of cores. Indexing large photo and video
+collections significantly benefits from fast, local SSD storage.
+
+