Update README.md

Author: single9

README.md

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+hashkey-middleware
+==================
+
+驗證使用權限的 Middleware。
+
+CLI Usage
+---------
+
+    node index.js [number]
+
+Hashkey
+---------
+
+- 用來使用 API 的鑰匙
+- 在生產環境中的有效時間為 15 分鐘
+- Package: [Hashids](https://www.npmjs.com/package/hashids)
+
+```js
+const Hashids = require('hashids/cjs');
+const hashSalt = process.env.SALT || 'my salt';
+const idHashids = new Hashids(hashSalt, 32, '0123456789abcdef');
+
+function genHashId(id = 141236) {
+    const time = Date.now();    // Timestamp
+    const idHash = idHashids.encode([id, time]);  // encode id and time to fake hex
+    return idHash;
+}
+
+// 編碼
+const idHash = genHashId();
+// 解碼
+const decodedId = idHashids.decode(idHash);
+
+console.log('Original ID: %d', id);
+console.log('Encoded ID: %s', idHash);
+console.log('Decoded ID: %j', decodedId);
+```