docs: populate readme file with project information

Update the project README file to include information on what the project
does and how to run it.

README.md

@@ -1,3 +1,123 @@
 # Fail2Ban Prometheus Exporter
 
 Go tool to collect and export metrics on Fail2Ban
+
+## Table of Contents
+1. How to use
+2. Docker
+    1. Environment variables
+    2. Docker run
+    3. Docker compose
+3. CLI usage
+4. Metrics
+
+## 1. How to use
+
+Run the exporter by providing it with a fail2ban database to read data from.
+Read access to the database is required.
+Once the exporter is running, metrics are available at `localhost:9191/metrics`.
+
+The default port is `9191`, but this can be modified with the `-port` flag.
+
+**Note:** By default fail2ban stores the database file at: `/var/lib/fail2ban/fail2ban.sqlite3`
+
+**Fail2Ban Jails**
+
+fail2ban can be configured to process different log files and use different rules for each one.
+These separate configurations are referred to as *jails*.
+
+For example, fail2ban can be configured to watch the system logs for failed SSH connections and Nextcloud logs for failed logins.
+In this configuration, there will be two jails - one for IPs banned from the SSH logs, and one for IPs banned from the Nextcloud logs.
+
+This tool exports several metrics *per jail*, meaning that it is possible to track how many IPs are being banned in each jail as well as the overall total.
+This can be useful to track what services are seeing more failed logins.
+
+## 2. Docker
+
+An official docker image is available on the Gitlab container registry.
+Use it by pulling the following image:
+
+```
+registry.gitlab.com/hectorjsmith/fail2ban-prometheus-exporter:latest
+```
+
+Use the `:latest` tag to get the most up to date code (less stable) or use one of the version tagged images to use a specific release.
+See the [registry page](https://gitlab.com/hectorjsmith/fail2ban-prometheus-exporter/container_registry) for all available tags.
+
+### 2.1. Volumes
+
+The docker image is designed to run by mounting the fail2ban sqlite3 database.
+The database should be mounted at: `/app/fail2ban.sqlite3`
+
+The database can be mounted with read-only permissions.
+
+### 2.2. Docker run
+
+Use the following command to run the forwarder as a docker container.
+
+```
+docker run -d \
+    --name "fail2ban-exporter" \
+    -v /var/lib/fail2ban/fail2ban.sqlite3:/app/fail2ban.sqlite3:ro \
+    -p "9191:9191"
+    registry.gitlab.com/hectorjsmith/fail2ban-prometheus-exporter:latest
+```
+
+### 2.3. Docker compose
+
+The following is a simple docker-compose file to run the exporter.
+
+```
+version: "2"
+services:
+  exporter:
+    registry.gitlab.com/hectorjsmith/fail2ban-prometheus-exporter:latest
+    volumes:
+    - /var/lib/fail2ban/fail2ban.sqlite3:/app/fail2ban.sqlite3:ro
+    ports:
+    - "9191:9191"
+```
+
+## 3. CLI usage
+
+```
+$ fail2ban-prometheus-exporter -h
+
+  -db string
+        path to the fail2ban sqlite database
+  -port int
+        port to use for the metrics server (default 9191)
+  -version
+        show version info and exit
+```
+
+## 4. Metrics
+
+Access exported metrics at `/metrics` (on the provided port).
+
+**Note:** All metric names include the `fail2ban_` prefix to make sure they are unique and easier to find.
+
+Exposed metrics:
+* `up` - Returns 1 if the service is up
+* `bad_ips` (per jail)
+    * A *bad IP* is defined as an IP that has been banned at least once in the past
+    * Bad IPs are counted per jail
+* `banned_ips` (per jail)
+    * A *banned IP* is defined as an IP that is currently banned on the firewall
+    * Banned IPs are counted per jail
+
+**Sample**
+
+```
+# HELP fail2ban_bad_ips Number of bad IPs stored in the database (per jail).
+# TYPE fail2ban_bad_ips gauge
+fail2ban_bad_ips{jail="jail1"} 6
+fail2ban_bad_ips{jail="jail2"} 8
+# HELP fail2ban_banned_ips Number of banned IPs stored in the database (per jail).
+# TYPE fail2ban_banned_ips gauge
+fail2ban_banned_ips{jail="jail1"} 3
+fail2ban_banned_ips{jail="jail2"} 2
+# HELP fail2ban_up Was the last fail2ban query successful.
+# TYPE fail2ban_up gauge
+fail2ban_up 1
+```