Add a notice about running migrations in multiple instances

Based on rubenv#58 and rubenv#45

Author: JekaMas

README.md

@@ -16,6 +16,7 @@ Using [modl](https://github.com/jmoiron/modl)? Check out [modl-migrate](https://
 * Up/down migrations to allow rollback
 * Supports multiple database types in one project
 * Works great with other libraries such as [sqlx](https://jmoiron.github.io/sqlx/)
+* Does not support running in concurrency or in multiple instances
 
 ## Installation