chore: add placeholder version in README.md file (#278)

Author: ti-chi-bot

README.md

@@ -63,4 +63,4 @@ $ bin/ng-monitoring-server --config config/config.toml.example
 $ pkill -SIGHUP ng-monitoring-server
 ```
 
-<!-- VERSION_PLACEHOLDER: v7.5.5 -->
+<!-- VERSION_PLACEHOLDER: v7.5.6 -->