nrf52dk: added examples

examples/nrf52dk/timer-test/README.md

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+Timers test
+===========
+Timers test is an application allows for testing clocks implementation for nRF52 DK. 
+The results of different tests are output to the console.
+
+There are 4 tests performed:
+
+	1) TEST clock_delay_usec() - measures duration of a NOP delay using rtimer. It's expected
+	                             that difference is close to 0.
+	                             
+	2) TEST rtimer - schedules an rtimer callback after 1 second. Prints actual time difference
+	                 in rtimer and clock ticks. It's expected that both values are close to 0.
+	                 
+	3) TEST etimer - schedules an event timer and measures time difference. It is expected that
+	                 the value is close to 0.
+
+The example requires one DK and it doesn't use SoftDevice. To compile and flash the
+example run:
+
+	make TARGET=nrf52dk timer-test.flash