Create MIGRATION.md

MIGRATION.md

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+## Migration From Other Enterprise Search
+
+### Google Search Apliance/Google Mini
+
+Fess provides GSA-Compatible API.
+The endpoint is localhost:8080/gsa.
+GsaApiManager is [here](https://github.com/codelibs/fess/blob/master/src/main/java/org/codelibs/fess/api/gsa/GsaApiManager.java).
+
+### Google Site Search/Google Custom Search
+
+See [JSON API](http://fess.codelibs.org/11.0/user/json-response.html).
+(We have a plan to provide Compatible API)
+
+### Others
+
+See [JSON API](http://fess.codelibs.org/11.0/user/json-response.html).