frontend: Add headlamp-events example plugin

Signed-off-by: Joaquim Rocha <[email protected]>

plugins/examples/headlamp-events/README.md

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+# Example Plugin: Reacting to Headlamp events
+
+Headlamp has the concept of Headlamp events, which are fired when something relevant happens in Headlamp.
+For example, when a user chooses to apply the changes their changing for a resource, or when a
+list/details view is loaded.
+
+To run the plugin:
+
+```bash
+cd plugins/examples/change-logo
+npm install
+npm start
+# Go to a list or details view to see one example of a Headlamp event.
+```
+
+- The plugin shows a notification whenever there is a Headlamp event, except for OBJECT_EVENTS type of
+  events (so checking the event type is demonstrated).
+- For events that have a resource, the resource name is shown.