docs(emqx_machine): Add readme

apps/emqx_machine/README.md

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+# EMQX Machine
+
+This application manages other OTP applications in EMQX and serves as the entry point when BEAM VM starts up.
+It prepares the node before starting mnesia/mria, as well as EMQX business logic.
+It keeps track of the business applications storing data in Mnesia, which need to be restarted when the node joins the cluster by registering `ekka` callbacks.