doc(plugin_hooks): Improve documentation for writing plugin hooks.

Author: NiklasMM

doc/en/writing_plugins.rst

@@ -621,12 +621,39 @@ the new plugin:
 
 Hooks are usually declared as do-nothing functions that contain only
 documentation describing when the hook will be called and what return values
-are expected.
+are expected. The names of the functions must start with `pytest_` otherwise pytest won't recognize them.
 
 For an example, see `newhooks.py`_ from `xdist <https://github.com/pytest-dev/pytest-xdist>`_.
 
 .. _`newhooks.py`: https://github.com/pytest-dev/pytest-xdist/blob/974bd566c599dc6a9ea291838c6f226197208b46/xdist/newhooks.py
 
+To register the hooks with pytest they need to be structured in their own module or class. This
+class or module can then be passed to the `pluginmanager` using the `pytest_addhooks` function
+(which itself is a hook exposed by pytest):
+
+.. code-block:: python
+
+    def pytest_addhooks(pluginmanager):
+        """ This example assumes the hooks are grouped in the pytest_hooks module. """
+        from my_app.tests import pytest_hooks
+        pluginmanager.add_hookspecs(pytest_hooks)
+
+To execute hooks in one of your fixtures, use the `pytestconfig` fixture. It holds all registered
+hooks under `hook` by their name.
+
+.. code-block:: python
+
+    @pytest.fixture()
+    def my_fixture(pytestconfig):
+        # call the hook called "pytest_my_hook"
+        # `result` will be a list of return values from all registered functions.
+        result = pytestconfig.hook.pytest_my_hook()
+
+Now your hook is ready to be used. To register a function at the hook, other plugins or users must
+now simply define the function `pytest_my_hook` with the correct signature in their `conftest.py`.
+
+.. note::
+    If a hook uses parameters only keyword arguments can be used to invoke it.
 
 Optionally using hooks from 3rd party plugins
 ---------------------------------------------