docs: add contributors (#636)

Author: Leko

README.md

@@ -6,3 +6,11 @@ JSConf Japan Website
 > The JSConf JP website has been migrated to Next.js since 2025.
 
 For more information, please [README in the 2025 directory](./2025/README.md).
+
+## Contributors
+
+<a href="https://github.com/jsconfjp/jsconf.jp/graphs/contributors">
+  <img src="https://contrib.rocks/image?repo=jsconfjp/jsconf.jp" />
+</a>
+
+Made with [contrib.rocks](https://contrib.rocks).