tests/periph_timer_cli: add README

tests/periph_timer_cli/README.md

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+# periph timer test
+
+This application provides a wrapper for testing the periph_timer api via the
+RIOT shell. To capture fast low level timing events some functions provide
+additional parameters to signal events over gpio pins e.g. before calling a
+timer function or on ISR callback execution.
+While this test can be run manually and with any external measurement equipment,
+it is recommended to use it together with the provided robot test on a PHiLIP-
+based HIL setup.
+Consult the 'help' shell command for available actions.
+
+Running this test with robot framework is done with this command:
+`BOARD=whatever PORT=/dev/ttyACM0 PHILIP_PORT=/dev/ttyACM1 DEBUG0_PORT=1 DEBUG0_PIN=3 make robot-test`
+
+Where DEBUG0_PORT and DEBUG0_PIN are the RIOT specific pin identifiers of the
+DUT pin that is connected to PHiLIPs DEBUG0 pin.