[stm32f4-discovery] Added timer example for the stm32f4 discovery board.

examples/stm32/f4/stm32f4-discovery/timer/README.md

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+# README
+
+This example demonstrates the use of timers to trigger an interrupt. This
+example will toggle two LEDs spelling out the following morse code:
+
+SOS -> ...---...
+
+Where dots are .1s, dashes .2s, gaps .05s and the word pause .5s.
+
+It's intended for the ST STM32F4DISCOVERY eval board. It should blink
+the LEDs on the board.
+
+## Board connections
+
+*none required*