message: Update lib.rs doc comment
diff: //! ## Custom Storage
  //! All you need to implement for `gluesql` is implementing 3 traits (2 for functions, 1 for
  //! running tests).
  +//! There is also an optional trait (AlterTable), whether implementing it or not is all up to you.
  //!
  //! * [Store](store/trait.Store.html)
  //! * [StoreMut](store/trait.StoreMut.html)
  +//! * [AlterTable - optional](store/trait.AlterTable.html)
  //! * [Tester](tests/trait.Tester.html)
  //!
  //! Custom storage examples to see,
  