Update primary and service readme files

Author: jburnett

README.md

@@ -4,6 +4,8 @@ The Bitcoin node metrics exporter for Prometheus based on Rust Bitcoin Community
 
 ## Install & Configure
 
+These instructions are for installing and running from the shell. To install as a SystemD service, follow [these instructions](./linux/SystemD-service.md).
+
 ### Install
 
 `cargo install btcnode-metrics`

linux/SystemD-service.md

@@ -23,6 +23,10 @@ Follow these steps to initialize the host before the first time running the serv
 
   `sudo cp ./linux/etc/systemd/system/btcnode-metrics.service /etc/systemd/system/`
 
+- Set the service to start on boot
+
+  `sudo systemctl enable btcnode-metrics`
+
 ## Updating
 
 Follow these instructions for updating the btcnode-metrics application or its config file.