Porting guide: Add Sleep API pg page

docs/contributing/sleep_deep_sleep.md

@@ -1,3 +1,52 @@
-### Sleep and deep sleep
+# Sleep HAL API
 
-[A document explaining sleep and deep sleep]
+mbed OS defines two sleep modes for HAL:
+
+- Sleep
+- Deep sleep
+
+Each target should document in their implementation:
+
+- The target's mode description for each mode (how target's mode map to mbed OS sleep modes)
+- Wake-up latency for each mode
+- Wake-up sources for each mode
+
+## Sleep
+
+The core system clock is disabled, both the low and high precision clocks are enabled, RAM is retained.
+
+1. Wake-up sources - any interrupt source should be able to wake-up the MCU
+1. Latency - should wake-up within 10 us
+
+## Deep sleep
+
+The core system clock is disabled. The low precision clocks are enabled, RAM is retained.
+
+1. Wake-up sources - RTC, low power ticker or GPIO should be able to wake-up the MCU
+1. Latency - should wake-up within 10 ms
+
+The deep sleep latency (10 ms) was chosen as the higher limit of the boards we support. Most of targets have wake-up latency for deep sleep within few microseconds, but often additional time is needed for reinitializing clocks or other configuration necessary to restore previous state.
+
+## Implementing the Sleep API
+
+There are two functions that target needs to implement to support sleep, their prototypes are located in [hal/sleep_api.h]():
+
+- Sleep
+
+```c++
+void hal_sleep(void);
+```
+
+- Deep sleep
+
+```c++
+void hal_deepsleep(void);
+```
+
+To enable sleep support in mbed OS `SLEEP` label needs to be added in `device_has` option of target's section in `targets.json` file.
+
+## Testing
+
+The [sleep HAL API test suite]() validates:
+- Sleep wake-up sources
+- Sleep wake-up latency