Create Power_capabilities.md to describe POWER class

Author: chipguyhere

docs/Power_capabilities.md

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+## Power management capabilities of M5Stack Core / Basic / Gray
+
+Document v0.1
+
+The power management capabilities are provided by the IP5306 power controller, specifically the IP5306_I2C version of the controller,
+that can be accessed on the I2C bus at address 0x75.  Access is possible using the `POWER` class,
+located in `Power.h` and `Power.cpp`, automatically included with `#include "M5Stack.h"`.
+
+### Capabilities
+
+- **Can the M5Stack Core turn itself off?** Yes, using `powerOFF()`, which shuts off the LED backlight, the WiFi radio, the
+     Bluetooth stack, and then asks the chip to remove the power before putting the ESP into deep sleep mode.  If called when the device is externally powered,
+     these actions (including the deep sleep mode) will give the appearance and effect of being powered off.  Pressing the power/reset button turns it back on.
+- **Can it wake itself back up after a pre-determined delay or at a specific time?**
+     No, since it doesn't have a real-time clock chip (RTC), but ESP32 sleep modes may provide a suitable alternative.
+- **Can it tell if it is running on external versus battery power?**
+- **Can it tell if the battery is being charged or discharged?**
+- **Can it tell if a battery is present or absent?** *possibly, but not formally*
+- **Can it tell the state of charge of the battery?** Yes, using `getBatteryLevel()`, which returns one of: 25, 50, 75, 100 (representing a percentage), or -1 (if unable to read)
+- **Can it read its battery voltage?** Not possible to read the voltage level directly, only the approximate percentage of 25/50/75/100.
+- **Can communication with the IP5306 power controller be tested and confirmed?** Yes, with `canControl()`
+- **What other capabilities?** *(to be answered -- translation of Chinese-language datasheet is difficult to understand)*
+  - *auto shutoff settings - voltage low for how long causes a shutoff*
+  - *inhibit the power button, report it as an event?*
+  - *setting the charging voltage and current*
+  - *distinguish between a software/hardware/watchdog/poweron reset*
+  - *maintain any of the settings across power cycles and/or complete loss of all power*