Add micro:bit timer examples and update todo

Added two Python scripts demonstrating the use of running_time() on micro:bit: one for printing elapsed time and another for a countdown timer with button interaction. Updated todo.md to include timers on micro:bit.

Author: DamoM73

python_files/running_time_1/main.py

@@ -0,0 +1,11 @@
+# running_time()
+from microbit import *
+
+'''
+The microbit has a very simple timing mechanism called running_time().
+It keeps a running count of the number of the time since
+the micro:bit started in milliseconds.
+'''
+
+while True:
+    print(running_time())

python_files/running_time_2/main.py

@@ -0,0 +1,34 @@
+# countdown timer
+from microbit import *
+import music
+
+'''
+To create a countdown timer, we will use a built-in function called running_time().
+running_time() keeps a running count of the number of the time since
+the micro:bit started in milliseconds.
+'''
+
+# --- SETUP
+# start components
+
+# store variables
+running = False
+timer = 5000
+
+# --- RUNNING
+while True:
+    # read sensor data
+    a_pressed = button_a.was_pressed()
+    
+    # process data
+    if a_pressed:    
+        running = True                          # start the countdown timer
+        start = running_time()                  # records the starting time
+    if running:
+        lapsed_time = running_time() - start    # calculates how long since button press
+        if timer - lapsed_time < 0:             # checks if countdown has reached 0
+            music.pitch(500,250, wait=False)
+            running = False                     # stops the timer
+    
+    # output data
+    

todo.md

@@ -1,3 +1,3 @@
 # To Do
 
-- [ ] Servo Drivers
+- [ ] Timers on micro:bit