docs/library/machine: Add docs for Counter and Encoder.

Add documentation for `machine.Counter` and `machine.Encoder` as currently
implemented by the esp32 port, but intended to be implemented by other
ports.

Originally authored by: Ihor Nehrutsa <Ihor.Nehrutsa@gmail.com> and
Jonathan Hogg <me@jonathanhogg.com>.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/library/machine.Counter.rst

@@ -0,0 +1,93 @@
+.. currentmodule:: machine
+.. _machine.Counter:
+
+class Counter -- pulse counter
+==============================
+
+Counter implements pulse counting by monitoring an input signal and counting
+rising or falling edges.
+
+Minimal example usage::
+
+    from machine import Pin, Counter
+
+    counter = Counter(0, Pin(0, Pin.IN))  # create Counter for pin 0 and begin counting
+    value = counter.value()               # retrieve current pulse count
+
+Availability: **ESP32**
+
+Constructors
+------------
+
+.. class:: Counter(id, ...)
+
+   Returns the singleton Counter object for the the given *id*. Values of *id*
+   depend on a particular port and its hardware. Values 0, 1, etc. are commonly
+   used to select hardware block #0, #1, etc.
+
+   Additional arguments are passed to the :meth:`init` method described below,
+   and will cause the Counter instance to be re-initialised and reset.
+
+   On ESP32, the *id* corresponds to a :ref:`PCNT unit <esp32.PCNT>`.
+
+Methods
+-------
+
+.. method:: Counter.init(src, *, ...)
+
+   Initialise and reset the Counter with the given parameters:
+
+   - *src* specifies the input pin as a :ref:`machine.Pin <machine.Pin>` object.
+     May be omitted on ports that have a predefined pin for a given hardware
+     block.
+
+   Additional keyword-only parameters that may be supported by a port are:
+
+   - *edge* specifies the edge to count. Either ``Counter.RISING`` (the default)
+     or ``Counter.FALLING``. *(Supported on ESP32)*
+
+   - *direction* specifies the direction to count. Either ``Counter.UP`` (the
+     default) or ``Counter.DOWN``. *(Supported on ESP32)*
+
+   - *filter_ns* specifies a minimum period of time in nanoseconds that the
+     source signal needs to be stable for a pulse to be counted. Implementations
+     should use the longest filter supported by the hardware that is less than
+     or equal to this value. The default is 0 (no filter). *(Supported on ESP32)*
+
+.. method:: Counter.deinit()
+
+   Stops the Counter, disabling any interrupts and releasing hardware resources.
+   A Soft Reset should deinitialize all Counter objects.
+
+.. method:: Counter.value([value])
+
+   Get, and optionally set, the counter value as a signed integer.
+   Implementations must aim to do the get and set atomically (i.e. without
+   leading to skipped counts).
+
+   This counter value could exceed the range of a :term:`small integer`, which
+   means that calling :meth:`Counter.value` could cause a heap allocation, but
+   implementations should aim to ensure that internal state only uses small
+   integers and therefore will not allocate until the user calls
+   :meth:`Counter.value`.
+
+   For example, on ESP32, the internal state counts overflows of the hardware
+   counter (every 32000 counts), which means that it will not exceed the small
+   integer range until ``2**30 * 32000`` counts (slightly over 1 year at 1MHz).
+
+   In general, it is recommended that you should use ``Counter.value(0)`` to reset
+   the counter (i.e. to measure the counts since the last call), and this will
+   avoid this problem.
+
+Constants
+---------
+
+.. data:: Counter.RISING
+          Counter.FALLING
+
+   Select the pulse edge.
+
+.. data:: Counter.UP
+          Counter.DOWN
+
+   Select the counting direction.