chore: add migration docs

migration.md

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+# Migration from version 2
+
+Migration from version 2 must be performed manually because the components library `@coreui/coreui-react` has been completely rewritten.
+
+The docs of the new components are available [here](https://coreui.io/react/docs/)
+
+The good news is that most probably it will be sufficient to migrate layout components (Sidebar, Header, Footer, Aside) and `Switch` component
+
+The best way to do a migration is:
+1. Install `@coreui/coreui-react` v3
+2. Make a copy of the current `containers` folder
+2. Paste [containers](https://github.com/coreui/coreui-free-react-admin-template/tree/master/src/containers) folder from v3 template to project
+3. Correct routing paths
+4. Add previous content to new template layout components
+5. Replace `Switch` components with `CSwitch`
+
+Layout components/ corresponding components in version 3
+- Aside -> CSidebar (with prop aside={true})
+- AsideToggler -> CToggler
+- Breadcrumb -> CBreadcrumbRouter
+- Footer -> CFooter
+- Header -> CHeader
+- HeaderDropdown -> CDropdown
+- NavbarBrand -> CSidebarBrand
+- Sidebar -> CSidebar
+- SidebarFooter -> CSidebarFooter
+- SidebarForm -> CSidebarForm
+- SidebarHeader -> CSidebarHeader
+- SidebarMinimizer -> CSidebarMinimizer
+- SidebarNav -> CSidebarNav + CSidebarNavDropdown + CSidebarNavItem
+- SidebarToggler -> CSidebarToggler
+- Switch -> CSwitch
+
+After the migration is done, you can start using new components of `@coreui/coreui-react` v3 library.