AP_HAL Readme: documentation for Scheduler added

README_AP_HAL.md

@@ -384,8 +384,27 @@ AP\_HAL::Scheduler
 --------------
 
 The `AP_HAL::Scheduler` class is pure virtual and can be found in
-`/libraries/AP_HAL/Scheduler.h`. 
-
+`/libraries/AP_HAL/Scheduler.h`.  The `AP_HAL::Scheduler` interface
+is designed to encapsulate both the timing utilities previously provided
+by the Arduino core (i.e. `millis` and `delay`), as well as scheduling
+asynchronous processes as a replacment to the ArduPilot `AP_PeriodicProcess`
+driver.
+
+The following methods are exposed by the `AP_HAL::Scheduler` interface:
+
+* `void init(void* implspecific)` : Initializer should setup all hardware.
+    This should be the very first initializer called by AP_HAL::HAL::init.
+* `void     delay(uint32_t ms)` : Duplicates Arduino core `delay`. Will call
+    delay callback, if registered.
+* `uint32_t millis()` : Duplicates Arduino core `millis`
+* `uint32_t micros()` : Duplicates Arduino core `micros`
+* `void     delay_microseconds(uint16_t us)` : Duplicates Arduino core `delayMicros`
+* `void     register_delay_callback(AP_HAL::Proc)` : Register a callback to be
+    used during calls to `delay`. Callback will be called at a 1ms period.
+* `void     register_timer_process(AP_HAL::TimedProc, uint32_t period_us, uint16_t phase)` : Duplicates `AP_PeriodicProcess::register_process`
+* `void     register_timer_failsafe(AP_HAL::TimedProc, uint32_t period_us)` : Duplicates `AP_PeriodicProcess::set_failsafe`
+* `void     suspend_timer_procs()` : Duplicates `AP_PeriodicProcess::suspend_timer`
+* `void     resume_timer_procs()` : Duplicates `AP_PeriodicProcess:resume_timer`
 
 
 Remaining ArduPilot AVR dependencies