message: Add SharedMemoryStorage to the src documentation comment
diff: //! Example code to see,
  //! * [tests/memory_storage.rs](https://github.com/gluesql/gluesql/blob/main/storages/memory-storage/tests/memory_storage.rs)
  //! * [tests/sled_storage.rs](https://github.com/gluesql/gluesql/blob/main/storages/sled-storage/tests/sled_storage.rs)
  +//! * [tests/shared-memory-storage.rs](https://github.com/gluesql/gluesql/blob/main/storages/shared-memory-storage/tests/shared_memory_storage.rs)
  //!
  //! After you implement `Tester` trait, the only thing you need to do is calling `generate_tests!` macro.
  