Add support for `add_index` and `drop_index` migration methods

I see that the only way to add an index to an existing table is to use the `alterIndex` method but this is **CRAZY** because it requires you to first drop the field and re-add it. 

This is crazy because you will lose whatever data is in that field.

This pull-request adds explicit migration methods for `add_index` and the complementary `drop_index`. There is also an updated `README` with usage of these new methods.

I see in the `README` this line: 

`Unit tests for new features and issues detected are mandatory to keep quality high. `

But I don't see any existing unit tests that I can reference for how to write a new one.