Add example for adding custom character

[LeCyberDucky]

Description

I have added a small example to the readme showing how to add custom characters.

Motivation and Context

So far, the syntax for adding characters isn't clear from the readme. This adds an example to make it easy for users add their own characters.

How Has This Been Tested?

I have only changed the readme. I have tested that the snippet actually does what it is supposed to and I have checked that the updated readme looks like it should.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• [x ] I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

I'm not sure if all of the above applies, since I didn't actually change code. I just added a snippet to the readme. I hope this can help somebody, since I had a bit of trouble working this out myself :)

[nhoizey]

Thanks a lot for this valuable addition! 👍

[nhoizey]

Fixes #100