Initial state is better included in the individual reducers

[rvvergara]

Hi @arielcamus,

In Milestone 4 , the initialState object to be included as individual 'initial states' in the respective reducers instead of having them in src/index.js. For instance in the book reducer we can have an initial state:

const initialState = [{<book1 object>}, {<book2 object>}]

This makes a cleaner index.js.

Also we should probably include here a step to put store creation in its own file (i.e, in a src/store/index.js file as it is a common practice in redux projects (at least in those projects I previously encountered). This helps the student to have a clearer way of structuring redux projects.