Create example for initial state

[nmay231]

Hello and welcome to Redux-undo! This issue is asking newcomers to open source for their help. If that's you then great!

We need good examples to show new users of the library how to, well... use the library!

The task is to create an example in docs/main/examples/ demonstrating how to set the initial state for a reducer. You can use whatever example state you wish, like a todo list for example. You can read up on the details in the README and also here (ignore the upgrading to 1.0 part).

Make sure to explain that you need to provide past, present, and future if you want to start with a history (explained in the second link).

If you are interested in doing this, reply to this issue. I'll get back to you to see if you have any questions and show you how to get started!

Instructions:

• Reply to this issue. And while you are waiting, you can...
• Read the contributing guide if you have not already (don't worry about the instructions for contributing code, this is a doc contribution).
• Since this is a relatively simple doc change, you can just create a fork using the button in the top right and make the file in your fork.
• Create the doc docs/main/examples/initial-state.md and show how to provide an initial state to the reducer.
• Link to it from docs/main/examples/readme.md and also docs/summary.md. These updates will be reflected on the gitbook site after merging (https://redux-undo.js.org).
• And feel free to ask questions! If you are ever uncertain about something, ask me here in this issue and I can help.

(Reminder for myself: #213 #214 )