Add info for disk usage check

Author: brablc

README.md

@@ -5,6 +5,7 @@ Detect unhealthy containers using two methods:
 1. **🚪 Opened ports/socks** - uses auto discovery and checks whether services with non zero replicas are available on those ports/socks.
 2. **📜 Docker events** - analyzes events generated by swarm when containers are created/destroyed 🔁 or have failing healthcheck 💔.
 3. **👨‍💼 Manager node count** - simply get number of manager nodes and alert when it is lower than expected.
+4. **📀 Disk usage** - basic disk usage check on all nodes and all volumes accessible to docker container.
 
 ## Configuration
 
@@ -46,7 +47,11 @@ When healtcheck fails for given number of retries, it would normally lead to a s
 ### 👨‍💼Manager node count
 
 When number of ready manager nodes is smaller than `SWARM_MANAGER_MIN`, it would generate an alert. Beware the alert may come from all manager nodes, where the alerter is running - now it is intentional, as the body of the alert might show which node disconnected itself.
-   
+
+### 📀 Disk usage check
+
+Use `DISK_USAGE_MAX` variable to get alert when the usage percentage gets exceeded on any volume accessible over the root / mount.
+
 ## Installation
 
 Add an alerter service to some of your stacks and add it to all networks where it should be checking ports:
@@ -64,6 +69,7 @@ services:
             mode: global
         environment:
             ALERT_SCRIPT: /app/integrations/zenduty.sh
+            DISK_USAGE_MAX: 85
             EVENTS_THRESHOLD: 3
             EVENTS_WINDOW: 60
             LOOP_SLEEP: 10
@@ -72,6 +78,7 @@ services:
             ZENDUTY_API_KEY: YOUR_ZENDUTY_API_KEY
         volumes:
             - /var/run/docker.sock:/var/run/docker.sock
+            - /:/rootfs:ro # For disk usage check
 ```
 
 ## Integrations