reference/cctype/isupper.md: Created sample implementation

Author: rotarymars

reference/cctype/isupper.md

@@ -50,3 +50,11 @@ isupper(' ') = 0
 isupper('\n') = 0
 isupper('0x0f') = 0
 ```
+
+## 実装例
+```cpp
+int isupper(int ch) {
+  return ch >= 'A' && ch <= 'Z';
+}
+```
+