Add FAQ entry for /healthz

Author: code-asher

doc/FAQ.md

@@ -19,6 +19,7 @@
 - [How does code-server decide what workspace or folder to open?](#how-does-code-server-decide-what-workspace-or-folder-to-open)
 - [How do I debug issues with code-server?](#how-do-i-debug-issues-with-code-server)
 - [Heartbeat File](#heartbeat-file)
+- [Healthz endpoint](#healthz-endpoint)
 - [How does the config file work?](#how-does-the-config-file-work)
 - [Blank screen on iPad?](#blank-screen-on-ipad)
 - [Isn't an install script piped into sh insecure?](#isnt-an-install-script-piped-into-sh-insecure)
@@ -240,6 +241,20 @@ If you want to shutdown `code-server` if there hasn't been an active connection
 you can do so by continuously checking the last modified time on the heartbeat file and if it is
 older than X minutes, kill `code-server`.
 
+## Healthz endpoint
+
+`code-server` exposes an endpoint at `/healthz` which can be used to check
+whether `code-server` is up without triggering a heartbeat. The response will
+include a status (`alive` or `expired`) and a timestamp for the last heartbeat
+(defaults to `0`).
+
+```json
+{
+  "status": "alive",
+  "lastHeartbeat": 1599166210566,
+}
+```
+
 [#1636](https://github.com/cdr/code-server/issues/1636) will make the experience here better.
 
 ## How does the config file work?