Update README.md

add info how to change start level

Author: marblexu

README.md

@@ -17,6 +17,11 @@ $ python main.py
 
 # How to Play
 * use mouse to collect sun, select the plant cards and seed the plant 
+* you can set the start level by changing START_LEVEL_NUM value in source/constants.py
+  * level 1 and 2：day level
+  * level 3: night level
+  * level 4: moving card select level
+  * level 5: wallnut bowling level
 
 # Demo
 ![demo1](https://raw.githubusercontent.com/marblexu/PythonPlantsVsZombies/master/demo/demo1.jpg)