Document the Adalogger class.

docs/adalogger.rst

@@ -1,2 +1,44 @@
 Adalogger
 ---------
+
+The Adalogger is a Featherwing with a PCF8523 RTC and a microSD card slot.
+
+* Header: ``feather/wing/adalogger.h``
+* Link: `Adalogger RTC + SD <https://www.adafruit.com/product/2922>`__
+* Update task: nothing is done
+
+The ``Adalogger`` is instantiated with one of two constructors:
+
+* ``Adalogger()`` is a wrapper for ``Adalogger(10)``, and sets up the
+  Featherwing with support for the onboard RTC and SD.
+* ``Adalogger(uint8_t cs)`` sets up the Featherwing with an alternate
+  SD CS pin if the default has been changed. Alternatively, using a
+  CS pin of 0 will disable SD card support.
+
+The ``setup`` method doesn't check whether the RTC has a valid date and
+time; this is provided by other functions.
+
+Note: if SD support is enabled, the card must be inserted and ready by the
+time ``setup()`` is called. There isn't support for hotswapping SD cards
+right now.
+
+SD support is provided by the `SdFat` external library; only FAT support is
+provided at this time.
+
+The ``Adalogger`` class provides the following methods for interacting
+with the SD card:
+
+* ``File openFile(const char *path, bool write)`` opens a file; the ``File``
+  type is provided by the `SdFat <https://platformio.org/lib/show/322/SdFat>`__
+  library. You don't need to include anything extra if you're not using
+  the SdFat library anywhere else.
+* ``bool exists(const char *path)`` returns true if the file or directory
+  named by ``path`` exists on the SD card.
+* ``bool remove(const char *path)`` returns true if the file named by ``path``
+  was successfully removed.
+* ``bool mkdir(const char *path)`` returns true if the directory named by
+  ``path`` was successfully created.
+
+This class is also an instance of the ``Clock`` class; in addition to the
+standard ``Clock`` methods, it has ``void adjustRTC(DateTime &dateTime)``
+to set the date and time in the RTC.