Add instructions to create migration

[alexmerlin]

Feature Request

Based on dotkernel/api#438:
Now that a migration file is no longer included in the repository, we must instruct developers to create them first, before executing them.

Q	A
New Feature	yes
RFC	no
BC Break	no

[alexmerlin]

Implemented in #134
Page is here: https://docs.dotkernel.org/api-documentation/v6/installation/doctrine-orm/#creating-migrations