Add Upgrade docs

doc/upgrade/2.0.md

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+# UPGRADE FROM 1.x to 2.0
+
+* Support for eZ Publish kernel 6.x has been dropped. Minimum supported version
+  is now 7.0
+
+* Support for PHP 5.x and 7.0 has been dropped. Minimum supported version is
+  now 7.1
+
+* All `*.class` parameters were removed from DI container. Use compiler passes
+  to set the new class or service decorators if you need to override any
+  services from legacy bridge.
+
+* Most of the services have been marked as private in Symfony DI container.
+  You should not fetch them from container directly any more, instead, inject
+  them as constructor arguments into your services. 
+
+* HTTP 404 error conversion in legacy mode `false` siteaccesses is now turned
+  on, without the possibility to turn it off. If you have any error pages
+  implemented in such siteaccesses in eZ Publish Legacy, you will need to
+  re-implement them in Symfony stack.
+
+* Deprecated Twig tag `ez_legacy_include` which rendered an eZ Publish Legacy
+  template from a Twig template has been removed. Use the native Twig `include`
+  tag instead.
+
+  Before:
+
+  ```
+  {% ez_legacy_include "design:my/old_template.tpl" with {"someVar": "someValue"} %}
+  ```
+
+  After:
+
+  ```
+  {% include "design:my/old_template.tpl" with {"someVar": "someValue"} %}
+  ```