docs: add @eslint/migrate-config to README (#60)

Author: mdjermanovic

README.md

@@ -9,6 +9,7 @@ This repository is the home of the following packages:
 -   [`@eslint/object-schema`](packages/object-schema)
 -   [`@eslint/config-array`](packages/config-array)
 -   [`@eslint/compat`](packages/compat)
+-   [`@eslint/migrate-config`](packages/migrate-config)
 
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