Add upgrade files

This adds UPGRADE-* documentation as done for Symfony. UPGRADE-2.0 documents BC breaks that will happen in 2.0 by removing deprecated functionality.

UPGRADE-1.11.md

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+UPGRADE FROM 1.10 to 1.11
+=========================
+
+PHP and Symfony version support
+-------------------------------
+
+ * Support for PHP 5.5, 5.6 and 7.0 was dropped
+ * Support for unsupported Symfony versions was dropped. The bundle now supports
+   Symfony 3.4 LTS and 4.1 or newer.
+ * Support for Twig 1.34 and below as well as 2.4 and below (for 2.x) releases
+   was dropped.
+
+Commands
+--------
+
+ * Deprecated instantiating `Doctrine\Bundle\DoctrineBundle\Command` without a
+   `ManagerRegistry` instance.
+ * Deprecated `setContainer` and `getContainer` in
+   `Doctrine\Bundle\DoctrineBundle\Command`.
+ * `Doctrine\Bundle\DoctrineBundle\Command` no longer implements
+   `ContainerAwareInterface`.
+
+Mapping
+-------
+
+ * Renamed `ContainerAwareEntityListenerResolver` to
+   `ContainerEntityListenerResolver`.
+
+Types
+-----
+
+ * Marking types as commented in the configuration is deprecated. Instead, mark
+   them commented using the `requiresSQLCommentHint()` method of the type.
+ * The `commented` configuration option for types will be dropped in a future
+   release. You should avoid using it.

UPGRADE-2.0.md

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+UPGRADE FROM 1.x to 2.0
+=======================
+
+PHP and Symfony version support
+-------------------------------
+
+ * Support for PHP 5.5, 5.6 and 7.0 was dropped
+ * Support for unsupported Symfony versions was dropped. The bundle now supports
+   Symfony 3.4 LTS and 4.1 or newer.
+ * Support for Twig 1.34 and below as well as 2.4 and below (for 2.x) releases
+   was dropped.
+
+Commands
+--------
+
+ * `Doctrine\Bundle\DoctrineBundle\Command` requires a `ManagerRegistry`
+   instance when instantiating.
+ * Dropped `setContainer` and `getContainer` in
+   `Doctrine\Bundle\DoctrineBundle\Command`.
+ * `Doctrine\Bundle\DoctrineBundle\Command` no longer implements
+   `ContainerAwareInterface`.
+
+Mapping
+-------
+
+ * Dropped `ContainerAwareEntityListenerResolver`, use
+   `ContainerEntityListenerResolver` instead.
+
+Types
+-----
+
+ * Marking types as commented in the configuration is no longer supported.
+   Instead, mark them commented using the `requiresSQLCommentHint()` method of
+   the type.
+ * The `commented` configuration option for types will be dropped in a future
+   release. You should not use it.