docs: add migration

Author: oklas

docs/migration.md

@@ -0,0 +1,24 @@
+# Migration
+
+# Migration from react-app-rewired-alias
+
+Dangerous mode is renamed to extended/extremal. The implementation
+for dangerous mode is moved to separaded liblrary with suffix **`-ex`**.
+
+* Replace library name for generic mode
+
+```diff
+- const {alias} = require('react-app-rewire-alias')
++ const {alias} = require('react-app-alias')
+```
+
+* Replace library name for dangerous mode,
+* Remove `/lib/aliasDangerous` path part from import,
+* Notice that library name is different (suffix `-ex` added),
+* Fangerous mode function `aliasDangerous` for `react-app-rewired` or `customize-cra` is renamed to `alias`.
+
+```diff
+- const {aliasDangerous} = require('react-app-rewired-alias/lib/aliasDangerous')
++ const {alias} = require('react-app-alias-ex')
+```
+