docs: application handler

Author: adhocore

README.md

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 - Command line application made easy
@@ -252,6 +253,25 @@ $app->add((new ConfigListCommand)->inGroup('Config'));
 ...
 ```
 
+#### Exception handler
+
+Set a custom exception handler as callback. The callback receives exception & exit code. The callback may rethrow exception or may exit the program or just log exception and do nothing else.
+
+```php
+$app = new Ahc\Cli\Application('App', 'v0.0.1');
+$app->add(...);
+$app->onException(function (Throwable $e, int $exitCode) {
+    // send to sentry
+    // write to logs
+
+    // optionally, exit with exit code:
+    exit($exitCode);
+
+    // or optionally rethrow, a rethrown exception is propagated to top layer caller.
+    throw $e;
+})->handle($argv);
+```
+
 #### App help
 
 It can be triggered manually with `$app->showHelp()` or automatic when `-h` or `--help` option is passed to `$app->parse()`.