Initial docs draft

Covers everything in the original pluggy.py module's doc string in much
more detail with links to external resources as seemed fit.

Resolves pytest-dev#14

Author: Tyler Goodlet

docs/api_reference.rst

@@ -1,11 +1,7 @@
-
-
 Api Reference
 =============
 
 .. automodule:: pluggy
     :members:
     :undoc-members:
     :show-inheritance:
-
-    

docs/calling.rst

@@ -0,0 +1,96 @@
+Calling Hooks
+=============
+The core functionality of ``pluggy`` enables an extension provider
+to override function calls made at certain points throughout a program.
+
+A particular *hook* is invoked by calling an instance of
+a :py:class:`pluggy._HookCaller` which in turn *loops* through the
+``1:N`` registered *hookimpls* and calls them in sequence.
+
+Every :py:class:`pluggy.PluginManager` has a ``hook`` attribute
+which is an instance of a :py:class:`pluggy._HookRelay`.
+The ``_HookRelay`` itself contains references (by hook name) to each
+registered *hookimpl*'s ``_HookCaller`` instance.
+
+More practically you call a *hook* like so:
+
+.. code-block:: python
+
+    import sys
+    import pluggy
+    import mypluginspec
+    import myplugin
+    from configuration import config
+
+    pm = pluggy.PluginManager("myproject")
+    pm.add_hookspecs(mypluginspec)
+    pm.register(myplugin)
+
+    # we invoke the _HookCaller and thus all underlying hookimpls
+    result_list = pm.hook.myhook(config=config, args=sys.argv)
+
+Note that you **must** call hooks using keyword `arguments`_ syntax!
+
+
+Collecting results
+------------------
+By default calling a hook results in all underlying :ref:`hookimpls
+<impls>` functions to be invoked in sequence via a loop. Any function
+which returns a none ``None`` result will have that result appended to
+a :py:class:`list` which is returned by the call.
+
+The only exception to this behaviour is if the hook has been marked to return
+its :ref:`firstresult` in which case only the first single none ``None`` value
+will be returned.
+
+.. _call_historic:
+
+Historic calls
+--------------
+You can invoke certain hooks with all call *history* provided to each *hookimpl*
+at register time. Put more simply, a *historic call* allows for all newly
+registered functions to receive all hook calls that happened before their
+registration.  The implication is that this is only useful
+if you expect that some *hookimpls* may be registered **after** the hook is
+initially invoked.
+
+Historic hooks must be :ref:`specially marked <historic>` and called
+using the :py:meth:`pluggy._HookCaller.call_historic()` method:
+
+.. code-block:: python
+
+    # call with history; no results returned
+    pm.hook.myhook.call_historic(config=config, args=sys.argv)
+
+    # ... more of our program ...
+
+    # late loading of some plugin
+    import mylateplugin
+
+    # historic call back is done here
+    pm.register(mylateplugin)
+
+Note that if you ``call_historic()`` the ``_HookCaller`` (and thus your
+calling code) can not receive results back from the underlying
+*hookimpl* functions.
+
+Calling with extras
+-------------------
+You can call a hook with temporarily participating *implementation* functions
+(that aren't in the registry) using the
+:py:meth:`pluggy._HookCaller.call_extra()` method.
+
+
+Calling with a subset of registered plugins
+-------------------------------------------
+You can make a call using a subset of plugins by asking the
+``PluginManager`` first for a ``_HookCaller`` with those plugins removed
+using the :py:meth:`pluggy.PluginManger.subset_hook_caller()` method.
+
+You then can use that ``_HookCaller`` to make normal, ``call_historic()``,
+or ``call_extra()`` calls as necessary.
+
+
+.. links
+.. _arguments:
+    https://docs.python.org/3/glossary.html#term-argument