Add page about systick and tickless mode

docs/contributing/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). You can use it to perform any task in the system, but for platforms using 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 you can override that using the [Tick API](http://arm-software.github.io/CMSIS_5/RTOS2/html/group__CMSIS__RTOS__TickAPI.html) that CMSIS-RTOS2 provides. In which case you'll need to provide your own source of the interrupts.
+
+## Tickless mode
+
+Tickless mode is an optimization mechanism available in RTOS for suspending the SysTick. You can use it in situations when RTOS is idle for multiple ticks, so you can achieve power savings by entering uninterrupted sleep. Target implementing tickless mode disables the SysTick, sets up wake-up timers and enters sleep mode when idle. It then exits sleep mode and re-enables the SysTick when the timer expires or some event occurs (like external interrupt).
+
+### Enabling tickless mode
+
+To support tickless mode in Mbed OS, your target needs to meet two requirements:
+
+- Support for Sleep HAL API
+- Support for Low Power Ticker HAL API
+
+To enable tickless mode support in Mbed OS, you need to add the `MBED_TICKLESS` macro in the `macros` option of the target's section in the `targets.json` file.
+
+Targets supporting tickless mode 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 users must not change the low power ticker interrupt handler when tickless mode is in use.
+
+### Testing
+
+There are no dedicated tests validating tickless mode. Running all Mbed OS tests suits, with particular focus on HAL sleep and HAL low power ticker tests, provides sufficient coverage.