Added basic documentation.

README.md

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+# IPsec Exporter [![CircleCI](https://circleci.com/gh/dennisstritzke/ipsec_exporter/tree/master.svg?style=svg)](https://circleci.com/gh/dennisstritzke/ipsec_exporter/tree/master)
+Prometheus exporter for ipsec metrics, written in Go.
+
+## Functionality
+The IPsec exporter is determining the state of the configured IPsec tunnels via the following procedure.
+1. Starting up the `ipsec.conf` is read. All tunnels configured via the `conn` keyword are observed.
+1. If the `/metrics` endpoint is queried, the exporter calls `ipsec status <tunnel name>` for each configured
+connection. The output is parsed.
+    * If the output contains `ESTABLISHED`, we assume that only the connection is up.
+    * If the output contains `INSTALLED`, we assume that the tunnel is up and running.
+    * If the output contains `no match`, we assume that the connection is down.
+
+## Value Definition
+| Metric | Value | Description |
+|--------|-------|-------------|
+| ipsec_status | 0 | The connection is established and tunnel is installed. The tunnel is up and running. |
+| ipsec_status | 1 | The connection is established, but the tunnel is not up. |
+| ipsec_status | 2 | The tunnel is down. |
+| ipsec_status | 3 | The tunnel is in an unknown state. |