doc: Update README.md

Author: OUDON

README.md

@@ -6,7 +6,7 @@ This is an rMQR Code image generator implemented in Python.
 ## Important Notice
 Please verify an image generated by this software whether it can decode correctly before use.
 
-## Usage
+## Basic Usage
 ```py
 from rmqrcode import rMQR
 import rmqrcode
@@ -31,6 +31,13 @@ The `fit_strategy` parameter is enum value of rmqrcode.FitStrategy to specify ho
 Here is an example of images genereated by each fit strategies for data `Test test test`:
 ![Example of fit strategies](https://user-images.githubusercontent.com/14174940/172822478-4f2b5fb8-49bd-464f-b6b2-c7347f71cbf5.png)
 
+## Advanced Usage
+### Select rMQR Code size manually
+To select rMQR Code size manually, use `rMQR()` constructor.
+```py
+qr = rMQR('R11x139', ErrorCorrectionLevel.H)
+qr.make("https://oudon.xyz")
+```
 
 ----
 The word "QR Code" is registered trademark of DENSO WAVE Incorporated.<br>