Add page about systick and tickless mode

Author: bulislaw

docs/advanced/systick_tickless.md

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+# SysTick and tickless mode
+
+## SysTick
+
+System tick timer (SysTick) is a standard timer available on most Cortex-M cores. Its main purpose is to rise an
+interrupt with set frequency (usually 1ms). It can be used to perform any task in the system, but for platforms
+utilizing RTOS, including mbed OS, it provides an interval for the OS for counting the time and scheduling tasks.
+
+mbed OS uses default SysTick source for most targets, but that can be overridden using the
+[Tick API](http://arm-software.github.io/CMSIS_5/RTOS2/html/group__CMSIS__RTOS__TickAPI.html) provided by CMSIS-RTOS2.
+
+## Tickless mode
+
+Tickless mode is an optimization mechanism available in RTOS for suspending the SysTick. It can be used in situations
+when RTOS will be idle for multiple ticks, allowing to achieve power savings by entering uninterrupted sleep. Targets
+implementing tickless mode will disable the SysTick, set up wake-up timer and enter sleep mode when idle. It will then exit
+sleep mode and re-enable the SysTick, when the timer expires or some event occurs (like external interrupt).
+
+mbed OS supports the tickless mode on multiple targets, they will define `MBED_TICKLESS`
+macro in [targets/targets.json](https://github.com/ARMmbed/mbed-os/blob/master/targets/targets.json).
+
+Targets supporting tickless mode will override the default SysTick mechanism and use
+[RtosTimer](https://github.com/ARMmbed/mbed-os/blob/master/rtos/TARGET_CORTEX/mbed_rtx_idle.cpp) implementation based on
+[low power ticker](https://github.com/ARMmbed/mbed-os/blob/master/drivers/LowPowerTicker.h).
+This change is necessary to avoid drift connected with using two different timers to measure time. It should be mostly
+invisible for users, except that the low power ticker interrupt handler shouldn't be changed when tickless mode is in use.