README: first draft

README.adoc

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+postfix_exporter
+================
+
+Collects continuesly events from postfix and dovecot via journald.
+Exports the collected event-counters and the currently count of queued messages.
+
+Usage
+-----
+
+For nix, `nix-shell` knows any dependencies.
+
+Any other:  You need `libffi` and some gems:
+
+	apt install libffi
+	bundle install
+
+Run puma for testing and use curl for first fetch.
+
+	puma
+
+	curl http://localhost:9296/metrics
+
+You should see the collected data.  On first run, this code be very empty.  Wait some minutes to have some emails.
+
+journald should not drop loglines!  TODO: /etc/systemd/journald.conf