[Docs][Plus] Add Plus upgrading guide

docs/book/installation/index.rst

@@ -10,5 +10,6 @@ The process of installing Sylius together with the requirements to run it effici
     installation
     sylius_plus_installation
     upgrading
+    sylius_plus_upgrading
 
 .. include:: /book/installation/map.rst.inc

docs/book/installation/map.rst.inc

@@ -2,3 +2,4 @@
 * :doc:`/book/installation/installation`
 * :doc:`/book/installation/sylius_plus_installation`
 * :doc:`/book/installation/upgrading`
+* :doc:`/book/installation/sylius_plus_upgrading`

docs/book/installation/sylius_plus_upgrading.rst

@@ -0,0 +1,39 @@
+.. rst-class:: plus-doc
+
+Upgrading Sylius Plus
+=====================
+
+Sylius regularly releases new versions, usually every two weeks.
+Each release comes with an ``UPGRADE.md`` file, which is meant to help in the upgrading process.
+
+1. **Update the Sylius Plus version constraint by modifying the ``composer.json`` file:**
+
+    .. code-block:: yaml
+
+        {
+            "require": {
+                "sylius/plus": "^0.21"
+            }
+        }
+
+2. **Upgrade dependencies by running a Composer command:**
+
+    .. code-block:: bash
+
+        composer update sylius/plus --with-all-dependencies
+
+    If this does not help, it is a matter of debugging the conflicting versions and working out how your ``composer.json``
+    should look after the upgrade.
+    You can check what version of Sylius is installed by running ``composer show sylius/plus`` command.
+
+3. **Follow the instructions found in the ``UPGRADE.md`` file for a given minor release.**
+
+   .. note::
+
+      As Sylius Plus is a private repository its README files (and CHANGELOG) have been exposed in a separate public
+      repository which can be found here: `<https://github.com/Sylius/PlusInformationCenter>`_
+
+
+.. image:: ../../_images/sylius_plus/banner.png
+   :align: center
+   :target: http://sylius.com/plus/?utm_source=docs