Add information about accessing TypoScript

The information about accessing TypoScript in extensions. This
patch adds the information for Extbase controllers (and refers
to existing documentation).

Information about how to access TypoScript outside of Extbase
controllers can be added in future patches.

Related: #463

Documentation/UsingSetting/AccessTypoScriptWithExtensions.rst

@@ -0,0 +1,66 @@
+.. include:: /Includes.rst.txt
+.. index:: TypoScript in extensions
+.. _extdev-access-typoscript:
+
+===================================
+Access TypoScript in your extension
+===================================
+
+.. note::
+
+   This part is written for extension developers.
+
+This page explains how to access TypoScript settings in extensions.
+
+Extbase controllers
+===================
+
+In Extbase controllers, :ref:`Flexform settings <t3coreapi:read-flexforms-extbase>` and TypoScript settings will be
+merged together. If a settings exists in both, the Flexform takes precedence (and overrides the TypoScript setting).
+Keep in mind that both Flexform and TypoScript settings must use the convention of preceding the setting with
+`setting.` (e.g. `settings.threshold`).
+
+Extbase offers some advantages: Some things work automatically out-of-the-box. However, you must stick to the
+Extbase conventions ("conventions over configuration").
+
+In order to access TypoScript settings from an Extbase Controller.
+
+.. rst-class:: bignums-xxl
+
+#. Use the convention of defining your TypoScript settings in :ts:`settings`
+
+   .. code-block:: ts
+
+      plugin.tx_myextension {
+         view {
+            # view settings
+         }
+
+         settings {
+            key1 = value1
+            key2 = value2
+         }
+      }
+
+#. Access them via :php:`$this->settings`
+
+   For example, in your controller:
+
+   .. code-block:: php
+
+      $myvalue1 = $this->settings['key1'] ?? 'default';
+
+.. seealso::
+
+   * :ref:`settings in Extbase extensions <t3extbasebook:configuration>`
+   * :ref:`Extbase TypoScript reference <t3extbasebook:typoscript_configuration>`
+
+Fluid
+=====
+
+If Extbase controllers are used, :php:`$this->settings` is automatically passed to the Fluid template. Thus, you can
+access settings like this:
+
+.. code-block:: xml
+
+   {settings.key1}

Documentation/UsingSetting/Index.rst

@@ -22,6 +22,7 @@ The TypoScript template configuration can be viewed and edited in the
 
    Entering
    AddTypoScriptWithExtensions
+   AccessTypoScriptWithExtensions
    Constants
    TheConstantEditor
    Register