Update README.md

Add basic description and usage

Author: shaoziyang

README.md

@@ -1,2 +1,34 @@
 # micropython-simple-scheduler
-micropython simple scheduler
+Simple task scheduler for micropython.
+
+- [time base version scheduler](tmScheduler)
+- [asyncio version scheduler](asScheduler)
+
+## uasge
+
+1. define user function.
+2. define task.
+3. add tasks to scheduler.
+4. run scheduler.
+
+
+```
+from scheduler import Scheduler, Task
+import machine
+
+def LED():
+    pyb.LED(1).toggle()
+
+def pn(n):
+    print(n)
+
+task1 = Task(LED, None, 1000)
+task2 = Task(pn, 1, 1000)
+task3 = Task(pn, 2, 1500)
+
+sc = Scheduler(machine.Timer(-1))
+sc.add(task1)
+sc.add(task2)
+sc.add(task3)
+sc.scheduler()
+```