docs: Add section explaining markdown options

[wolfiediscord]

Description

Currently, in the README file, there is no section that explains the different markdown options for notes, warnings, important, etc. While it seems self explanatory, new contributors must examine every page of documentation or dig into the code to see all available options, which can be discouraging or create badly formatted pages.

Potential Fixes

• Add a section to the README.md that explains these options or links to a guide
• Create a new file that explains these options and provides some general markdown instructions for new contributors

With these fixes, it would make contributing new games, (and potentially applications), much easier.

[ethanuppal]

These docs aren't sufficient? https://crates.io/crates/mdbook-callouts

[wolfiediscord]

Those docs are pretty good, but they aren't mentioned anywhere in the contributing guidelines. Adding them as just a quick link would work.

[mattmaniak]

Addressed in #189. Feel free to check the diff and suggest changes @wolfiediscord.