Add missing registration step in the extension docs

Change-Id: I07bf315db590a6460eb22a9b9eee9e9b8720e2d7

src/main/resources/Documentation/extensions.md

diff --git a/src/main/resources/Documentation/extensions.md b/src/main/resources/Documentation/extensions.md
index ccc07c7..6477b39 100644
--- a/src/main/resources/Documentation/extensions.md
+++ b/src/main/resources/Documentation/extensions.md
@@ -54,6 +54,11 @@
 }
 ```
 
+Finally, you will need to register your plugin's healthcheck in the plugin's `Module` class:
+```java
+DynamicSet.bind(binder(), HealthCheck.class).to(FooHealthCheck.class);
+```
+
 To build the plugin in the gerrit-CI, as
 [documented](https://gerrit-review.googlesource.com/Documentation/dev-plugins.html#_cross_plugin_communication)
 by gerrit, you should be configuring your build job as follows: