Add migrate.md

Author: ReneWerner87

MIGRATE.md

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+This document contains instructions for migrating to various storage versions.
+
+### 0.1 -> 0.2
+v0.2 fixes [a bug](https://github.com/gofiber/fiber/issues/1258) in MYSQL, Postgres and Arangodb in which 
+inserting non-UTF8 characters would trigger a panic due to the values being saved in a TEXT column instead of a 
+BYTEA/BLOB column. Migration instructions (note you may need to adjust the table names if you have supplied a custom
+config to the storage):
+
+**Postgres**
+```sql
+ALTER TABLE fiber_storage
+ALTER COLUMN v TYPE BYTEA USING v::bytea;
+```
+
+**MYSQL**
+```sql
+ALTER TABLE fiber_storage MODIFY COLUMN v BLOB;
+```
+
+**Arangodb**
+
+No migration other then updating the library is necessary.