Document ip range reserved by CircleCI

We use this IP range for dedicated network link on machine executor

Author: kimh

jekyll/_cci2/executor-types.md

@@ -253,6 +253,8 @@ jobs:
 The `image` key is not supported on private installations of CircleCI.
 See the [VM Service documentation]({{ site.baseurl }}/2.0/vm-service) for more information.
 
+The IP range `192.168.53.0/24` is reserved by CircleCI for the internal use on machine executor, so please avoid using the range in your jobs.
+
 ## Using macOS
 {: #using-macos }