feat(am): add note into migration guide about the new way to boostrap…

… the administrator user

Fixes gravitee-io/issues#3975

pages/am/3.x/installation-guide/installation-guide-migration.adoc

@@ -10,6 +10,15 @@ WARNING: If you plan to skip versions when you upgrade, ensure that you read the
 
 WARNING: Be sure to run scripts on the correct database since `gravitee-am` is not always the default database! Check your db name by running `show dbs;`
 
+
+== Upgrade to 3.10.0
+
+=== Management API
+
+With previous versions the administrator user for the Management API was created on startup using hard coded values for username and password. Starting from 3.10.0 it is possible to define your own values thanks to the `security.providers` section into the `gravitee.yaml` file of the Management API. By default, this configuration section will be apply only during the first start of AM. If you want to apply this configuration to other starts, you have to enable the `refresh` option for the provider. (`refresh` is disabled by default)
+
+WARNING: If you enable the refresh option after an AM upgrade from 3.9.x or earlier, be careful to the password encoding algorithm option that should be set with the same value as the one use for the existing inline provider. By default the yaml file set this value to BCrypt.
+
 == Upgrade to 3.9.0
 
 NOTE: For more information about the breaking changes of this version please visit link:/am/current/am_breaking_changes_3.9.html[Breaking changes in 3.9]