Add note for index requirement with FK constraint
Add note for index requirement with FK constraint

Update migration_style_guide.md

mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq