docs: add explanation for startupExceptionHandler (#241)

README.md

@@ -467,6 +467,7 @@ The `UnleashConfig$Builder` class (created via `UnleashConfig.builder()`) expose
 | `unleashContextProvider`                   | An [Unleash context provider used to configure Unleash](#2-via-an-unleashcontextprovider).                                                                                                                                                       | No       | `null`                                                                                                               |
 | `unleashFeatureFetcherFactory`             | A factory providing a FeatureFetcher implementation.                                                                                                                                                                                             | No       | [`HttpFeatureFetcher::new`](src/main/java/io/getunleash/repository/HttpFeatureFetcher.java)                          |
 | `unleashMetricsSenderFactory`              | A factory providing a MetricSender implementation.                                                                                                                                                                                               | No       | [`DefaultHttpMetricsSender::new`](src/main/java/io/getunleash/metric/DefaultHttpMetricsSender.java)                  |
+| `startupExceptionHandler`                  | Handler for the behavior in the event of an error when starting the client.                                                                                                                                                                      | No       | `null`                                                                                                               |
 
 When you have set all the desired options, initialize the configuration with the `build` method.
 You can then pass the configuration to the Unleash client constructor.