docs: Add support for emoji reactions to posts.

[MichelMajdalani]

What is done? As part of the documentation (walkthrough) for a first core change, I added the required code changes a new user needs to perform to add support to emoji reactions to posts.

Why is it done? The documentation for making the first core change is missing in the docs.

How is it tested? This is still WIP. It hasn't been tested.

Linked discussion:

• Documentation PR: https://github.com/deso-protocol/docs/pull/50