tests: add README for xtimer_drift

tests/xtimer_drift/README.md

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+# xtimer_drift test application
+
+Make note of the PC clock when starting this test. Let it run for a while, and
+compare the printed time against the expected time from the PC clock. The
+difference is the RIOT timer drift, this is likely caused by either:
+
+- an inaccurate hardware timer, or
+- bugs in the software (xtimer or periph/timer)
+
+This test will run a periodic timer every `TEST_INTERVAL` microseconds (`TEST_HZ`).
+The current time will be printed once per second, along with the difference
+between the actual and expected `xtimer_now` value. The first output variable
+`drift`, represents the total offset since start between `xtimer_now` and the
+expected time. The second output variable `jitter`, represents the difference
+in drift from the last printout. Two other threads are also running only to
+cause CPU load with extra interrupts and context switches.