docs: add note about required strictNullChecks option (#800)

* docs: add note about required `strictNullChecks` option

* Update README.md

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>

* Update README.md

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>

Author: wolfy1339

README.md

@@ -718,6 +718,8 @@ In addition to these types, `@octokit/webhooks` exports 2 types specific to itse
 
 Note that changes to the exported types are not considered breaking changes, as the changes will not impact production code, but only fail locally or during CI at build time.
 
+**⚠️ Caution ⚠️**: Webhooks Types are expected to be used with the [`strictNullChecks` option](https://www.typescriptlang.org/tsconfig#strictNullChecks) enabled in your `tsconfig`. If you don't have this option enabled, there's the possibility that you get `never` as the inferred type in some use cases. See [octokit/webhooks#395](https://github.com/octokit/webhooks/issues/395) for details.
+
 ### `EmitterWebhookEventName`
 
 A union of all possible events and event/action combinations supported by the event emitter, e.g. `"check_run" | "check_run.completed" | ... many more ... | "workflow_run.requested"`.