Add Mysql 8 workaround

source/developers-guide/shopware-5-upgrade-guide-for-developers/index.md

@@ -75,9 +75,38 @@ on upgrade until Shopware 5.6, but they won't be kept in sync anymore.
 
 For a complete overview check the **Removals** part of the <a href="https://github.com/shopware/shopware/blob/5.5/UPGRADE-5.5.md#removals">Upgrade.md</a>. 
 
+### MySQL 8 workaround
+
+Due to a mixture of MySQL 8 and Doctrine constraints, the column `s_core_documents.ID` will be renamed to
+`s_core_documents.id` on the fly if MySQL 8 is being used. To be able to do that, the service `\Shopware\Components\Compatibility\LegacyDocumentIdConverter`
+was introduced, which is checked in the file `engine/Shopware/Models/Order/Document/Document.php` to determine if
+a Doctrine model with uppercase or lowercase `id` needs to be used.
+
+If you need to reference this column in your own model, we recommend to use the same workaround there. You can use the
+same service (see above) with id `legacy_documentid_converter` for that.
+
+The reason for this workaround is that MySQL 8 forces ids in foreign key constraints to be lower case.
+
+This is a problem in current systems since we have an uppercase `ID` in table `s_order_documents`.
+MySQL doesn't care if we use `ID` in the table and `id` in the constraint, but Doctrine needs both to be written 
+in the same way. On new installations of Shopware 5.5 this is already the case, both are lowercase there.
+
+So in order to support MySQL 8 on updates from older Shopware versions we need to change the case of the `id` column
+in `s_order_documents`, which breaks support of blue/green deployments as older versions of Shopware (< 5.5) need
+that column to be uppercase.
+
+Since this change is only really necessary if you are using MySQL 8, it is only enforced when a MySQL 8 server is
+detected. A downgrade to an older Shopware installation wouldn't be possible anyway in that case, as Shopware 5.4
+does not support MySQL 8 yet.
+
+If you want to make this migration offline, there is the command `sw:migrate:mysql8` to check if the migration was
+executed and do so if you want.
+
+The column `s_core_documents.id` will be lowercase from Shopware 5.6 forward.
+
 ### Library updates
 
-- Updated `Symfony` to version 3.4.14 LTS. Some nice, new features are <a href="https://symfony.com/blog/new-in-symfony-3-4-simpler-injection-of-tagged-services" target="_blank">Simpler injection of tagged services</a> or <a href="https://symfony.com/blog/new-in-symfony-3-4-lazy-commands" target="_blank">Command Lazyloading</a>.
+- Updated `Symfony` to version 3.4.15 LTS. Some nice, new features are <a href="https://symfony.com/blog/new-in-symfony-3-4-simpler-injection-of-tagged-services" target="_blank">Simpler injection of tagged services</a> or <a href="https://symfony.com/blog/new-in-symfony-3-4-lazy-commands" target="_blank">Command Lazyloading</a>.
 - Updated `jQuery` to 3.3.1.  You can see the <a href="https://jquery.com/upgrade-guide/3.0/" target="_blank">jQuery update guide</a>
     for a list of important and breaking changes and links to the migration plugin.
     In Shopware, the relevant changes were: