[README] Renamed all readme files to indicate markup format.

examples/stm32/l1/stm32l-discovery/button-irq-printf/README.md

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+* Prints to the screen when the button is pushed/released (irq driven)
+  115200@8n1 console on PA2 (tx only)
+* uses basic timer 6 with overflows to generate a 1ms counter (not an ideal
+  use, but shows some api methods and can be demoed on the disco board)
+* uses basic timer 7 with the exti interrupts to do ghetto input capture.
+  Not as fast or precise as the real input capture modes, but can be used
+  on any gpio pin.
+
+No effort at saving power is made here. Current consumption on the Disco board
+is ~7.5mA when the green tick led is off, and about 10.5mA when it is on.
+
+example output:
+
+hi guys!
+TICK 0
+TICK 1
+TICK 2
+Pushed down!
+held: 443 ms
+Pushed down!
+TICK 3
+held: 217 ms
+Pushed down!
+held: 99 ms
+Pushed down!
+TICK 4
+held: 73 ms
+Pushed down!
+held: 60 ms
+TICK 5
+Pushed down!
+held: 98 ms
+Pushed down!
+
+