Add demo gif to the readme

[wargio]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
• I've updated the Rizin book with the relevant information (if needed).

Detailed description

Adds the demo GIF to the readme.

[notxvilka]

1. Let's keep just one
2. We probably need to record a new one, showing more than just one particular command, a brief showcase of multiple features at once.

[notxvilka]

I am against this particular example - it's too simplistic and gives a wrong expectation about the tool