Added fs_promise_demo.js to show how to use fs.promise API

reference/fs_promise_demo.js

@@ -0,0 +1,35 @@
+const fs = require('fs').promises;
+const path = require('path');
+
+// Create folder, this is using asynchronous Promise file system API
+fs.mkdir(path.join(__dirname, 'test'))
+    .then(() => console.log('Folder created...'))
+    .catch(error => {
+        if (error.code === 'EEXIST') {
+            console.log('Folder already exists...');
+        } else { throw error; }
+    });
+
+const filePath = path.join(__dirname, 'test', 'hello.txt');
+
+// Create and write to file
+fs.writeFile(filePath, 'Hello World!')
+    .then(() => console.log('File written...'))
+    .then(() =>
+
+        // File append
+        fs.appendFile(filePath, '\nI love NodeJS')
+    ).then(() => {
+
+        // Read file
+        fs.readFile(filePath, 'utf-8')
+            .then(data => console.log(`\n"${data}"\n`));
+    }).then(() =>
+
+        // Rename file
+        fs.rename(
+            filePath, 
+            path.join(path.dirname(filePath), 
+            'helloworld.txt')
+        ).then(() => console.log('File renamed...'))
+    );