cmd/makemac: update readme with service file info

Add instructions for updating the systemd service file on the host.

Change-Id: Ifb73845c32992cea58b3bd734b2980fcb231c0f9
Reviewed-on: https://go-review.googlesource.com/55814
Reviewed-by: Sarah Adams <shadams@google.com>
diff --git a/cmd/makemac/README.md b/cmd/makemac/README.md
index 3ab1806..a38671d 100644
--- a/cmd/makemac/README.md
+++ b/cmd/makemac/README.md
@@ -4,7 +4,7 @@
 
 The makemac command starts OS X VMs for the builders.
 
-## Deploying
+## Deploying `makemac`
 
 ```
 * On Linux,
@@ -17,3 +17,16 @@
   * sudo systemctl restart makemac
   $ sudo journalctl -f -u makemac     # watch it
 ```
+
+## Updating `makemac.service`
+
+```
+* On Linux,
+  $ scp -i ~/.ssh/id_ed25519_golang1 cmd/makemac/makemac.service gopher@macstadiumd.golang.org:makemac.service
+
+On that host,
+  $ sudo mv makemac.service /etc/systemd/system/makemac.service
+  $ sudo systemctl daemon-reload
+  $ sudo systemctl restart makemac
+  $ sudo journalctl -f -u makemac     # watch it
+```