fix #10 - add final README covering the topic

Author: dysen

deploy/alpine-mirror/README.md

@@ -0,0 +1,6 @@
+# How to Use
+
+- Adjust ./synchroniser/crontab for desired schedule of synchronising
+- Adjust ./synchroniser/mirror.sh for which repos to exclude, etc.
+- Image has to be re-built whenever crontab or mirror.sh are changed
+- Run by `podman compose up` or `docker compose up`