Add docs for using promises with fs.read() & fs.write()

README.md

@@ -128,7 +128,7 @@ Methods
 - [writeJsonSync](docs/writeJson-sync.md)
 
 
-**NOTE:** You can still use the native Node.js methods. They are promisified and copied over to `fs-extra`.
+**NOTE:** You can still use the native Node.js methods. They are promisified and copied over to `fs-extra`. See [notes on `fs.read()` & `fs.write()`](docs/fs-read-write.md)
 
 ### What happened to `walk()` and `walkSync()`?
 

docs/fs-read-write.md

@@ -0,0 +1,39 @@
+# About `fs.read()` & `fs.write()`
+
+[`fs.read()`](https://nodejs.org/api/fs.html#fs_fs_read_fd_buffer_offset_length_position_callback) & [`fs.write()`](https://nodejs.org/api/fs.html#fs_fs_write_fd_buffer_offset_length_position_callback) are different from other `fs` methods in that their callbacks are called with 3 arguments instead of the usual 2 arguments.
+
+If you're using them with callbacks, they will behave as usual. However, their promise usage is a little different. `fs-extra` promisifies these methods like [`util.promisify()`](https://nodejs.org/api/util.html#util_util_promisify_original) (only availible in Node 8+) does.
+
+Here's the example promise usage:
+
+## `fs.read()`
+
+```js
+// Basic promises
+fs.read(fd, buffer, offset, length, position)
+  .then(results => {
+    console.log(results)
+    // { bytesRead: 20, buffer: <Buffer 0f 34 5d ...> }
+  })
+
+// Async/await usage:
+async function example () {
+  const { bytesRead, buffer } = await fs.read(fd, buffer, offset, length, position)
+}
+```
+
+## `fs.write()`
+
+```js
+// Basic promises
+fs.write(fd, buffer, offset, length, position)
+  .then(results => {
+    console.log(results)
+    // { bytesWritten: 20, buffer: <Buffer 0f 34 5d ...> }
+  })
+
+// Async/await usage:
+async function example () {
+  const { bytesWritten, buffer } = await fs.write(fd, buffer, offset, length, position)
+}
+```