Create INSTALL_PFSENSE.md

As requested by @AnalogJ

docs/INSTALL_PFSENSE.md

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+# pfsense Install
+
+This bascially follows the [Manual collector instructions](https://github.com/AnalogJ/scrutiny/blob/master/docs/INSTALL_MANUAL.md#collector) and assumes you are running a hub and spoke deployment and already have the web app setup.
+
+
+### Dependencies
+
+SSH into pfsense, hit `8` for the shell and install the required dependencies.
+
+```
+pkg install smartmontools
+```
+
+Ensure smartmontools is v7+. This won't be a problem in pfsense 2.6.0+
+
+
+### Directory Structure
+
+Now let's create a directory structure to contain the Scrutiny collector binary.
+
+```
+mkdir -p /opt/scrutiny/bin
+```
+
+
+### Download Files
+
+Next, we'll download the Scrutiny collector binary from the [latest Github release](https://github.com/analogj/scrutiny/releases).
+
+> NOTE: Ensure you have the latest version in the below command
+
+```
+fetch -o /opt/scrutiny/bin https://github.com/AnalogJ/scrutiny/releases/download/vX.X.X/scrutiny-collector-metrics-freebsd-amd64
+```
+
+
+### Prepare Scrutiny
+
+Now that we have downloaded the required files, let's prepare the filesystem.
+
+```
+chmod +x /opt/scrutiny/bin/scrutiny-collector-metrics-freebsd-amd64
+```
+
+
+### Start Scrutiny Collector, Populate Webapp
+
+Next, we will manually trigger the collector, to populate the Scrutiny dashboard:
+
+> NOTE: if you need to pass a config file to the scrutiny collector, you can provide it using the `--config` flag.
+
+```
+/opt/scrutiny/bin/scrutiny-collector-metrics-freebsd-amd64 run --api-endpoint "http://localhost:8080"
+```
+> NOTE: change the IP address to that of your web app
+
+### Schedule Collector with Cron
+
+Finally you need to schedule the collector to run periodically.
+
+Login to the pfsense webGUI and head to `Services/Cron` add an entry with the following details:
+
+```
+Minute: */15
+Hour: *
+Day of the Month: *
+Month of the Year: *
+Day of the Week: *
+User: root
+Command: /opt/scrutiny/bin/scrutiny-collector-metrics-freebsd-amd64 run --api-endpoint "http://localhost:8080" >/dev/null 2>&1
+```
+> NOTE: `>/dev/null 2>&1` is used to stop cron confirmation emails being sent.