docs: remove dead warning/critical keys from ping_monitor config example

These fields were never read by the plugin; thresholds are configured
server-side. Also document the -b flag in README.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -507,6 +507,9 @@ hbc --boot your-server.example.com
 
 # Verbose output
 hbc -v your-server.example.com
+
+# Send 'boot' and 'shutdown' messages on start and exit 
+hbc -b your-server.example.com
 ```
 
 You can also run it via the module entrypoint:

hbd/client/plugins/ping_monitor.py

@@ -13,12 +13,8 @@ plugins:
     count: 3              # ICMP packets per ping run (default 3)
     timeout: 5            # seconds before a host is considered unreachable (default 5)
     hosts:
-      8.8.8.8:
-        warning: 20.0     # ms
-        critical: 100.0   # ms
-      192.168.1.1:
-        warning: 5.0
-        critical: 20.0
+      - 8.8.8.8
+      - 192.168.1.1
 ```
 
 Reported metrics per host (metric key uses the hostname with dots/colons replaced