Add a MIGRATION.md file which has migration instructions from 7.0.0 to 8.0.0

Author: oguzkocer

MIGRATION.md

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+###From `7.0.0` to `8.0.0`
+
+We are no longer pushing to remote after creating a new commit or a branch. That means, developers need to manually push the changes or add push commands in the project's `Fastfile`. Most importantly, we can no longer immediately trigger beta/final builds after creating a new commit because the changes will not be in remote yet. If you want to keep the existing behavior, you'll need to add a push command before these triggers.
+
+For example, in [WordPress-Android's `new_beta_release` lane](https://github.com/wordpress-mobile/WordPress-Android/blob/0c64cb84c256e004473e97d72b4ac6682ebc140b/fastlane/lanes/release.rb#L86), we download translations, bump the beta version and then trigger a new build in CI. After migrating to `8.0.0` of `release-toolkit`, we'll need to add [`push_to_git_remote`](https://docs.fastlane.tools/actions/push_to_git_remote/) command before this trigger to keep the existing behavior.