Added exceptional Migration Guide for v9.2

docs/migration/MigratingFromV9.1ToV9.2.md

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+Because the V9.2 is **completely** backward compatible with the V9.1, there's a small guide to help you to migrate your code:
+
+### :warning: Phpfastcache core drivers has been moved to their own extensions
+If you were using one of those drivers:
+
+`Arangodb`
+`Couchdb`
+`Dynamodb`
+`Firestore`
+`Mongodb`
+`Solr`
+
+Your **CODE IS SAFE AND FINE**, you just need to add a new composer dependency, ex: 
+```bash
+composer install phpfastcache/arangodb-extension
+```
+
+### :new: Couchbasev4 has been as an extension:
+```bash
+composer install phpfastcache/couchbasev4-extension
+```
+However `Couchbasev3` **will stay in the core** for compatibility reasons but will be deprecated.
+
+
+That's it :)
+------
+More information in our comprehensive [changelog](./../../CHANGELOG.md).
+
+
+
+