docs: mention missing OverlayProvider requirement (#196)

* docs: mention missing `OverlayProvider` requirement

* docs: Update README-ko_kr.md

---------

Co-authored-by: Yongbeen Im <[email protected]>

Author: scarf005

README-ko_kr.md

@@ -14,6 +14,19 @@ npm install overlay-kit
 
 ## 예제
 
+먼저, `OverlayProvider`를 추가해주세요:
+
+```
+import { OverlayProvider } from 'overlay-kit';
+
+const app = createRoot(document.getElementById('root')!);
+app.render(
+  <OverlayProvider>
+    <App />
+  </OverlayProvider>
+);
+```
+
 ### 간단한 오버레이 열기
 
 `overlay.open`을 사용하면 오버레이를 간단하게 열고 닫을 수 있어요.

README.md

@@ -14,6 +14,19 @@ npm install overlay-kit
 
 ## Example
 
+First, add provider:
+
+```tsx
+import { OverlayProvider } from 'overlay-kit';
+
+const app = createRoot(document.getElementById('root')!);
+app.render(
+  <OverlayProvider>
+    <App />
+  </OverlayProvider>
+);
+```
+
 ### Opening Simple Overlays
 
 You can easily open and close overlays using `overlay.open`.